On the car - If you don't take the 40, you have to give it to me because you don't deserve it. If you make sure it's in good shape and bring any likely spares before you leave (which you should do anyway), then you have no reason to need to get cruiser parts in Mexico, and as other have said once you're past Mexico you're home free in cruiser country. I'm about to limp my ailing 60 back up through Mexico and if that old girl can do it so can yours.
The 40 will be the lowest profile, and although the 05 ford is a good option and would also blend in ok, you could have a disaster with it if you go to any but the best mechanic in the capital of whatever country you are in.
Yeah you can do without A/C, but on some days it's nice to have. I say if your truck has it, take the belt off until you really need it. If not, you just have to be sweaty that's all. Being smelly helps prevent robbery.
If you were to take an american truck, use a box truck shell with windows for a camper instead of a standard camper back and nobody will know you're a rich, stupid tourist
With that $5k-$7k instead of buying another truck that will be "ok", you could make your 40 really sweet and have a great living space.
On minimalism - the more space you have, the more crap you fill it with. Leave with an empty truck and after a few months you'll find it filled with all sorts of junk you bought along the way. Plan your living space practically, and then at least the junk that fills up your truck will add to your comfort instead of just being a pain in the ass. I left with a coleman stove, hiking gear, and milk crates full of clothes and cooking wear. My camping setup isn't much better than that now, and it kind of sucks... I have learned from this mistake - minimalism only lasts as long as you can do it for, plan around that!
A roof top tent is used in some countries to keep the lions below, but in other places it is an item of convenience. You can open it quickly and sleep on your truck rather than having to set up a tent or move around your gear inside the truck.
Jerry cans are a must IMHO (I used the one I always keep full just today, and thus didn't have to backtrack 10km to fill up and waste 45 minutes, miss the good weather, and miss seeing the volcano I drove 4 hours to see). I'm not going to store mine inside because I don't have the extra space, it's not that necessary, they don't make me stand out that much having them on the outside (I have cheap ones, not flashy ones), and lastly I don't like breathing in diesel fumes all day for months.
On selling the car - I'm pretty sure not all countries in central america put it in your passport? Maybe I'm wrong, I'll have to ask my girlfriend. As has been said, it's not going to be straightforward or legal though, you can get in trouble, screw yourself bigtime, and it's not a good plan to leave home with.
