I have been travelling now 3 years (roughly 160 000km) with my 1200GS.
Bike has now around 330 000km on it, so starts to be properly broken in.

You can find BMW parts from central and south america (bigger cities).
Of course, it is better to carry some spare parts (spark plugs, brake pads etc) with you.

I would say it is better to get a bike you are comfortable travel with, preferably bike that you can service and fix by yourself (saves a lot of money).

There is no one "right" bike, people travel with all sort and size bikes.
Main thing is that bike should fit you and your way of travel.

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Sounds to me like you've enough off road experience on your GSA do get it through 95% of S. America. Of course there are positives and negatives to BIG vs. Smaller bikes. But for TWO UP, hard to beat a BMW GS.

That said, I'd guess about half of ride reports I've read over last 10 years involve some type of failure/breakdown. Bringing spares is wise ... if you can dig into the trans, final drive, CAN bus or ABS ... then get it all back together.

Most can't so it ends up in a truck for a ride to the BMW workshop.

Common things are easy to carry: brake pads, plugs, filters et al. But more complex failures may require packing bearings and such ... but knowledge and skill are what's really important when doing repairs.

Typically ONE BMW dealership in each country ... Brazil probably has more (in Rio & Sao Paulo). Some are good ... some have horror stories regards quality of repairs and time taken to get parts from Germany.

But all bikes can break down or have crash damage ... and parts can be hard to get for any bike, unless it's a bike sold
locally.

A well prepped GS should be fine. And given your previous experience with it ... I'd bet you'll be OK. Always good to ride a bike you're familiar with and trust.
Good planning!