Completely agree with Frank. Don't even think about trying to import a bike into another country. Having done a temporary import into Oz, I would say, further, don't even think about trying to import a bike into Australia.
We bought a KLR250 in Perth and it lasted very well all the way to Argentina, where we sold it on - your trip in reverse Michelle. It's a great choice for South America. Superb. Not just for smaller people either - I would (at 84kg) choose it for myself.
I would fit an screen, as you will be doing lots of miles. You can pick these up dead cheap in most South American countries.
Can't help you on the differences of models, but I wouldn't worry about it.
Around the world 2000-2004, on a 1993 Honda Transalp