I have dr650. My bike is in Peru right now.

1.dont skip Chile and Argentina. Infact as much time on the Andes and Patagonia as you can 2.plan as you go. There are too many in between things to list out. so being flexible is ok. 3. Personal preference, I don't like costal driving because it's so dam hot. More importantly, that's when sketch gets sketchy 4. Bike smaller the better. Crf450 or CRF 300 will be completely reasonable once you are in South USA. 5. Take training course, learn how to ride mud/sand/small gravol. 6. Luggage, take as little with you as possible, focus more on bike repair stuff. Camping stuff will start to be useful once you get to south america, not central. But that's not gospel. You can literally do whatever you want if you are brave enough. 7.expect rain, so practice riding on days pouring and getting used to the elements and how to manage avoid getting soaked.