I'd suggest the XR400 if you really want to get into the mountains, the pavement doens't last long. But you're right, it is a long way - otoh, are you in a hurry?
Read Ryan and Dan's account of their trip - same route on CB550's. It's in the Travellers Stories section.
The "road" varies from excellent to absolutely terrible - you, your interests, and your experience and bike choice will dictate where you turn back.
You can go all the way to Ushuaia on pavement if you're lucky, if not, well it can get interesting when the weather turns bad. Read my account of Ecuador at
http://www.horizonsunlimited.com/johnson/Ecuador.shtml. Thsis should give you some idea of the bad end of the scale.
An XR appropriately loaded (LIGHTLY) will be a blast on the backroads, easy and fun. BUT you will have fewer creature comforts. And ride slower on the good roads - but that's not a bad thing.
Have fun!
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Grant Johnson
Share the Dream!
at:
www.HorizonsUnlimited.com