A lot of 'right-on' opinions here. You can't buy experience. My only experience is with an 2001.5 2nd gen. Dodge .75t. It's the extra cab version with suicide doors and we have gone a trip of 4 hrs. with 6 full sized folks. As the driver, I had a great time, but for the 3 in the back seat, not so much. If you are not going to use it much, a gasser might be a good solution. You must drive forever with a diesel to get back your investment at purchase of 4-6K $ just for the engine.
For years and years all my Ford and GM friends have said my VP44 will go south; my lift pump will fail; and the truck will basically fall apart. Well, wonder of wonders, it's still in one piece after nearly 150K miles. Just lucky i guess. I still like it. Why? The 2-gen HO is a good motor and I moved the lift pump off the engine and down on the frame in front of the fuel tank. It becomes a pusher, not a sucker. Less heat/less vibration. Replaced with a much more robust pump. Have two spares under the driver's seat. The biggest cause of failure of the VP44 is moments after the lift pump has given up the ghost, not before. It must have fuel. Have a fuel pressure IDIOT light that shines brightly when the fuel pressure gets below 5 pounds. No wire/no engine upgrades. Oh, i did fiddle with the waste gate to get the turbo to kick in sooner. But that only makes the engine run hotter. The NV5600 manual trans. The very last of the cast iron box trannies. 360 pounds, wet. No shaft or bearing deflection here. The box is rated at 26000 pounds max. There is no big jump between gears like on an NV4500. I have found no hill or situation where I could not select a 'just-right' gear, whether running empty or at the CGVW of the rig. 19,300 pounds. It shifts very smoothly with synchros all around. (rumor has it) that parts are hard to come by? Maybe, but so far so good, but i do have a source, if need be. Original trans and clutch. NV241HD t-case. What's the big deal? It's the 'snow plow prep' T-case. It is strength rated the same as the non-HD, but you have to look farther to see why. What is the biggest cause of T-case failure while plowing? Shock loading. The HD has a much wider chain to absorb all that guff, but has the same rear output bearing as the non-HD. So, the weak link is the output bearing which doesn't take it in the shorts like the chain does. This is the year they put the first 35 spline Dana 80's in 3/4 t. Dodge/Cummins/HO's (except for the V-10's). I have never gotten the reported 20 mpg that the 12V's get, but I have been in the 19 mpg range a lot (empty). With my little Lance hard side (10,200 pounds, wet) i get between 11 and 15 mpg on the road going no faster than 64 mph with no wind and no grade with 3.55 gears. 11 is when i'm in the hills against the wind. 15 is no wind no grade. Wind resistance with the hard side on makes the most difference in mpg. Basically, nothing has failed on my Dodge besides the usual replaceable suspects. (batteries/tires) One interior light doesn't work. Original unit bearings in front. (how could that happen?) I did add the steering brace between the frame members clamped to the steering box. It made a difference, but not all the difference. 3 inch lift pucks on the front coils.
My brother has a 1999 Ford F250 P-stroke and he likes it a lot. With an OUTFITTER! pop up camper and towing his rock crawler in a big enclosed trailer around it does very well. He does have a wire and went thru a couple 6th gear jobs before he lightened up when in 6th and has had no trouble lately. He is one of the very few that have had woes with the aluminum German 6 speed gear box. It could be because he exceeded the CGVW rating time after time running over 20,000 pounds to get to the trailhead. My bro is 'JR' on the Pirate page.
If i had a choice and money were no object, i would still gravitate toward the simplest, most over-built drivetrain i could find that would run on Mexican diesel fuel. That narrows the search down to prior to 2007, i believe.
Just a little more first hand experience for your hard drive. Good luck in your search.
regards, as always, jefe