Just stumbled upon this and enjoyed reading through. We will add our 2cents worth in case its helpful to anyone. We haven't yet made it to SA, so i cant speak to what changes after the darien gap (but it seems increasingly likely that we may make the jump).
A month at home has given us plenty of time to evaluate our setup and the pros/cons before we hit the road again...which might explain what i just realized is a pretty long post (sorry).


After reading all previous pages, i would reiterate things mentioned in earlier posts:
- After all, it's just stuff (but we would also cry for a day over our hard drive)

- Less=More

- Balance
We aren't the lightest travelers out there and we aren't the cheapest, but we are probably not far behind in either category and still manage to have an AMAZINGLY good time. To us, that's all about Balance.
When we first went backpacking years ago we packed 3 bottles of wine (in the bottle) for the 12miles in. We never gave up packing the wine (and bourbon)...but we finally found the right amount for the length of our trips and decided to carry it in bladders...
Happy hour yes, but happier hours on the trail. also Yes. =)


This is very similar to our packing decisions on the road.
Essentials/Must Haves
Vehicle- 

This is probably everyone biggest decision. Those of you that have been to our blog know we've had our issues and breakdowns...but we wouldn't change a thing.

Bipbop said it, but to reiterate- notice nobody here has mentioned a specific vehicle or unimog as being needed. It isnt.
You CAN do this drive in any vehicle already parked outside your house and anyone who tells you that you differently either hasn't done it before or is trying to sell you something before you go. =)

Our rig is 45years old, barely ran when we left, ran on only 2cylinders most of our trip, never had anything close to 4wd, has a laughable ground clearance...
and it may well be the perfect rig for this or any similar trip. Our bus gave us the ability to go anywhere, sleep anywhere and do anything really without limits. It also made us tons of friends, could be fixed by anyone (including -now- us) and gave us lots of stories to tell.
We did more miles of offroad than any other travelers we've met- AND we lived, ate and slept in more comfort than most.


Our recommendation: buy a van- better yet, buy a vw bus.

Awning-
Shade is a must have unless you plan on staying in hotels every night. We almost never did. Our first awning was the bus depot EZup, and it worked fine, but luckily we saw the speed of the arb on someone else's rig before leaving town...
The
arb awning is the difference between suffering inside because its too much trouble, and lounging outside in the shade.
We never really felt the need for screened walls but have screens on the Bus so didnt need them. The awning got rolled out almost everytime we stopped. I only wish i had rigged a small on on the back for shade while engine tuning =)

Screens-
We made our own with some fabric and snaps. Nothing fancy but absolutely critical. If i had it to do again id use something not quite so bugproof but a bit more accesible to wind/draft. No seeums were rarely the issue and normal screen fabric would have ben fine and allowed a bit more airflow.
Fans- 

see screens and airflow above. Our fan ran every night, all night, no exceptions. The problem was we felt bad for the dog and it usually faced her. I would have installed more fans and cared less about what they looked like. Comfort lets you live for cheap and have a good time. Heat was our number one enemy in the trip so far.
Water- 

Um...without it, you die. 'Nuf said.
We have a 12gal tank with a pump/faucet (no sink). The ability to have plenty of water is key and this setup worked great for us.
If i were designing from scratch, i would have just installed a pump that i could toss into a garrafon, (since thats what we all buy on the road) and thrown it under a cabinet. No pouring and no storing the extra tank...but then again i would want more than 5gallons, so i guess d build room in front/behind it for a second. =)

The waste involved alone would make us do this. we can count on one hand the number of plastic bottles we had to throw away last year, but the number we see most tourists/traveler burn through turns our stomaches.
Fridge/Ice-

We disagree with most posters here and say take the frig- and before i start on details, we spent almost every night sleeping and cooking/eating inside the vehicle. If taking a fridge means you cant sleep inside your vehicle maybe you should get a different rig. =)

We loved having ice cold cocktails every night and we made almost all of our own meals. Neither could be possible without the fridge and all travelers we ran into were possibly happier to see our ice trays than to see us. We bought (because were cheap) the truckfridge and it works sufficiently enough. Next time id splurge for the
ARB or another top-loader as its more efficient. Without our hog of a frig we would almost never have to plug in or run the engine.
We also didn't want to leave a beach or a secret spot until we decided to leave, not when we had to in oder to get new food or recap the battery/frig, so we highly recommend solar.
Extra fuel-
We had 5gal of gas in plastic Sceptre tanks at all times and used it often. It might have simply been because we never had a working fuel gauge, and for the last 9 months also had neither a working speedometer or odometer. Regardless...it happens.
Propane/Cooktop-
We installed a 2 burner SMEV cooktop. Brilliantly compact and worked like a champ.
We also carry a
clear/full sized propane bottle. First, we only refilled it because a propane truck rolled right up beside us. Second, you cant refill the small green bottles anyway (and, for the record you also cant recycle them- they ALL end up not biodegrading in your local landfill). Our propane also has a long hose to fuel our marine grill and works in a pinch for a ceramic heater we keep buried (havent been close to needing it in mexico/central america...maybe if we make the jump to south america)
Air Compressor-
Our cheap Harbor Freight compressor saved us more times than i can count. The ability to air down on rutted dirt roads or to get out of the sand when needed and then air back up on the highway was huge. It got a gremlin (or died, i dont know yet) during our last few days and those were the only time i ever feared going offroad.
Tow Strap-
Okay, we both hope you use it less than we did (almost impossible not to)...but even a
cheap tow strap saved us when nothing else could. Not much good to have a useful stranger if they cant do anything to help you!
Recommended
Good solar setup-
We loved having energy at our fingertips and it makes the fridge decision easy. Worth the money and we know several travelers who bought both fridge and solar en route.
Grill-

We could have gotten by with only our cooktop but probably got more good use out of the grill. Fresh seared ahi from the fisherman who just walked up the beach, Burgers and steaks that the coyboy just shaved off at the carniceria...you'll wish you had the ability to grill if you dont, but yes...you can survive without it. We bought a marine grill to hold up to the elements...no complaints yet.
jumper cables-
We never needed for us (had to push start plenty, but that was a starter issue not a battery one). Our jumper cables made us local heroes many days over. And who knows...one day we might need them too.
portable shower-
Friends gave us their old
camp shower before leaving and we were glad it was in the back. The ability to stay clean is important (maybe), but the ability to stay cool is huge. A quick rinse off does wonders to combat 100+degree heat.
sand ladders(depending on your offroad use)-

We had another traveler bring us a set of the cheap(er)
smitty bilts before the maya rally in mexico. I cant say we needed them much after mexico, but if you plan on a lot of offroading, especially in mexico (and even moreso in baja (the road-lacking east cape drive for example) a set of sand rails is great.
Not a necessity as you'll get out eventually because locals are friendly enough to help you....but especially those of you who chose not to listen to our solar/frig advice will wish you could have gotten out without waiting (see kiwi-yanks post). We also love sleeping on the beach, which entices you to pull ever so much closer to the water, which means- eventually...you'll get stuck.
spanish/local languange- jen's spanish was pretty rough when we left and mine was almost nonexistant...but 6 months in we were both mostly fluent. I never truly felt integrated into local culture until we did so in their language... 

You will make it through without learning the local language, but you'll always just feel like an outsider. I would have focused more on learning before we left knowing what i do now. This will also save you a bundle in everything from buying veggies off the truck to crossing borders.
Completely Unnecessary
gps/spot/phone- 

GPS- We had no GPS or phone and never wanted one. The few times we followed someone were some of our few (almost) dangerous situations of the trip. They always seem to take you into a ravine or into oncoming traffic on a one-way street.

Spot- I keep reading that you should buy a spot because help will come eventually...
Im not sure what kind of problem you plan on having en route, but i can assure you 911 isnt coming south of san diego...you're more likely to get the help you need from a local stranger/family (and i doubt you'll have their number programmed in).
Also...i read about more spots dying/malfunctioning en route than any single other object. In my opinion, not a great record for the system you bought to save you in an emergency.
Phone- We were online constantly...but to me a phone equalled being on-call like i was the last 15 years. We skyped when we needed to call and/or text, which was rare. IF youre planning to use a phone id pickup a card in each country vs paying for roaming...but if youre leaving to disconnect like we did- leave it at home!!
Wetsuit/Snorkel gear-
only if spearfishing. we never used our wetsuits and only pulled the snorkel set out when fishing. Havent yet convinced outrselved to throw them out as we dont really need the space for anything else...but also not sure why they are in there.
4wd- 

Thats right. See vehicle above...completely unnessesary, at least this side of the Darien Gap. I cant speak to the other.
RTT- 

This was really tempting when we discussed taking our old vehicle and for a time, even with the bus, but were really glad we didn't have it on top.

 We got pretty hot many nights inside the bus (and thats mostly because we went through baja in august/september...which isnt really recommended), but for the flexibility/ability to camp anywhere anytime without needing to drive 2 hours after dark to reach that campground you planned- totally worth leaving it off your list.
You can't really hide it en route meaning you can't really ever stealth camp unless you saved the sleeping space inside, and then im not sure why you have it to begin with. We met overlanders who are returning home to lose the RTT and build a bed inside before they continue...id far prefer the freedom of sleeping in the bed of a truck to lugging a RTT, but in a van you get the best of both worlds.

Sink-

Many told us to leave this out, was a huge concern for me- but they were all spot on. We use our faucet all the time, but have never wanted or needed a sink. We did have a foldable plastic tote/tub for dishes...god add on.
Backpacking Tent- 

Only once when someone came to visit did we even talk about wanting one, and it's far easier for your guest to check it than for you to carry it around for a year =)
Weapons-
Its worth putting although i hope nobody actually has this on your list. The closest thing we have to protection is a can of wasp spray, and its only been used when ants tried taking over the bus in california. A weapon is highly unlikely to get you out of trouble, but can certainly get you into some.
