Trek 520 and LHT have proven themselves to be reliable touring bikes under nearly every imaginable condition. They come equiped with mid grade, readily available components that can often be found in exotic locations. The LHT at least has 26" wheel option. Don't think the 520 does. Stick with 26" and have the spokes stress relieved and maybe upgraded. The wheel build is critical for an epic tour. At $1500, you've reached the point of diminishing return for the $$$.

Consider a disc brake on the rear and a rim brake on the front. Two discs are unnecessary, adding complexity and weight. By mixing you get great stopping power in wet/muddy conditions, rim preservation, and the field repair simplicity of a rim brake. Nobody makes a bike this way so you'd have to modify whatever. I think the LHT may have, or soon will have, a disc option. Also, stick with friction shifters, again for reliability and simplicity. That's what you need to shoot for on a long tour in developing countries.