Definitely go with 26 inch wheels; stay away from brifters. Use bar end shifters that can also work in a friction mode. Keep all the parts simple and strong. Where you are going, I'd carry xtra spokes and all the tools needed to break down the bike. I'm not sure what kind of touring bike you are thinking about getting but both Surly and Soma make good quality steel frames with 26 inch wheels that are designed for touring; there are others. Carry a spare tire (maybe two), carry a lot of spare tubes and of course patches. As long as you can get the wheels rolling, you can make it into a town where eventually you can get the parts you need.
I don't know how good a mechanic you are but you need to know how to build and break down your bike from one end to the other. In your case, it makes sense to buy a frame (like the Soma or the Surly) and build it up from the ground up (you don't need to build your own wheels but you do need to know how fix them). If funds are limited, go with good cup and cone hubs like the shimano mtn bike hubs laced onto stout rims. They are easy to fix and find parts for. If you have the money, I'd go with really high quality sealed hubs and BBs like Phil Woods.
I really like Latin America as well so I'm jealous; this will be a great trip but be safe and plan ahead.
Edit: while I have never done a bike trip in Latin America, I know the region and have done extensive bike touring throughout the US and Europe. I also worked as a mechanic when I was in college. Feel free to PM if you want to talk about your trip.