Great recommendation Maria. The little Yamaha is a good one if set up right and a better seat is sourced. (Seat Concepts, Corbin, Sargent, Renazco and a few more
possibles)
Nick, what bike are you riding? Most all the good 250's can cruise at 100 km. all day and still get great fuel economy.
What is your budget? Both the KLR250 or Super Sherpa can be good travel bikes if well set up. I would NOT buy an old, high mile beater. Spend a bit more, get a nice lo mile bike. It will take a huge beating going Canada to Tierra del fuego.
I'm saving up now to buy a 250cc bike, after doing over 60,000 miles (90K km)
on my DR650. The DR650 has been great but it's HEAVY and now pretty much worn out.
I'm buying a Yamaha WR250R. Might be too tall for your girl friend. Also take close look at Honda CRF250L. Cheaper than Yamaha and ... BTW ... of Japanese brands ... HONDA has the MOST coverage throughout S. America.
Ask anyone who's done the ride.
IMO, Maria's XT250 suggestion is still the best. You can buy a NEW ONE or find a really nice used one. NO BEATERS for S. America.
Good luck, have fun! S M I L E !!!