I'll suggest that a service body is HEAVY. Staying light and minimizing overall vehicle height while maximizing ground clearance for a a setup that is basically an expedition vehicle is what you should be honing in on, and you're on the right track.
Of course buying already setup flatbed possibly with a couple tool boxes should be more economical than building it. Think ranching areas for buying a used flatbed. MT, WY, eastern wa, anywhere rural out west has a lot of flatbed trucks.
Idk how mechanically inclined you are, but on one hand, staying simple with a 7.3 Powerstroke or a 12V or 24v Cummins is IMO the best for mechanical and electrical simplicity, but the older the truck gets the more "stuff" gets wore out.
I'd be looking for a lighter weight aluminum sided hard side TC, flatbed preferably aluminum but those are more expensive and the tool boxes, if not already setup, I'd wait til you have the camper and flatbed setup and then match the boxes. You can get fairly economical generic side boxes for not too much $.
Expedition to South America, while I've never been there, I'd think srw truck would be largely preferable. Get a 250 or 350 (2500 or 3500), doesn't matter. You'll need to upgrade the rear suspension anyway and the rest of the truck is the same regardless of model and since you're looking at old trucks, miles and condition is far more important than the badge on the fenders. Those are only ther to provide something to debate on a camper forum! Lol
While I'd be tempted to get 19.5 tires and rims for capacity and durability, they are extremely rough riding and hard on suspension components on the front of a vehicle that isn't heavy enough to need them, on unimproved roads. And you lose the ability to air them down compared to E load tires. Ideally, 19.5s or some big super singles in the back and normal tires up front, but that creates a mismatch and the need for 2 types of spares. This I'd do some serious research on and shoe the truck with what's available south of the equator in case you shred a tire or 2.
Good luck with your build and trip. BTW, if you're in the PNW currently, there's a basically brand new 7.3 Powerstroke for sale on Craigslist Seattle. But they want $27k for it.