Hmm...
I say do the preventative maintenance; fill her with gas...grab the passports and head South. Heck...I'd take your rig tomorrow; as is. (Yes, I'm simplifying things. There will be other serious planning that will need to go into a long term trip like what you're talking about, but the rig is pretty simple, actually.)
There will be no perfect vehicle other than the one you've got....you and your significant other will have a great time. Any issues with the rig will...I repeat...will, get sorted out and you'll do fine. Yes...you may have to wait for parts...and customs...but that'll give you time to hang out and make friends.
Some of my best stories are when I'm on the side of a road in the middle of nowhere Mexico, broken down, and the next thing I know someone is stopping to assist...I've even been offered some coffee whilst I worked....
For a DII I would, if I had to plan it...the basics:
Change, and keep the old stuff as spares: Water pump, fluids, cooling system, 02 sensors; other known problem areas. (maybe carry some parts that are known to possibly fail; head gasket?)
Spare fuses/bulbs, electrical bag/tools, etc.
OME Medium/heavy springs/shocks...cheap for what you get.
New tires of stock size or slightly larger; but not much....: BFG have never failed me...one spare
Remove the rear seat; maybe put a cargo floor in with tie downs if the stock ones are not sufficient.
High lift jack & ARB recovery kit; full sized shovel.
Sounds like ya'll will have a great time. Let us know what your route is...and if you want company for part of the trip...there are plenty of us who just like to wander around....
Cheers, and let me know if I can be of assistance.
-H-