Rachel,
If you are really looking for something that will ALWAYS have parts ALL over Central and South America and is small enough for you to reach the ground on (and pick up when it falls over), then you want the Tornado.
I am 6'3" and 100 kg and ride a Honda XR650L, but due to this concern for economy and parts, my son and I decided to take the Honda XR250 Tornado (made in Brazil, but sold EVERYWHERE in C and SA).

Buy it in Mexico and go south!
Our trip was only 10,000 km
Around the Block 2007 | but we had
minimal concerns and found cheap parts (stuff that normally does wear) at almost every stop. For spares we carried: a spark plug, an extra tube in each size and 2 smaller carburetor jets for the altitude. That's it!
The stock tank has a 180 km range, and the few times we needed more, we carried a small container trapped to our luggage.

We started with soft side bags, but due to the rough roads and them tearing up; ended up with a single rear rack lockable box and a small backpack strapped to the tank, which worked great! (this is in La Paz)
The power was sufficient to climb to 16,000 ft and pass cars at the same time. Any more is just luxury and a temptation to overload the bike with too much stuff.