I love S. America. I have never bike toured there but have traveled for multiple weeks. Most of my time has been in Chile or Colombia, both wonderful countries.
Chile, while not expensive is not cheap. Their wine is very inexpensive and delicious. They have some incredible scenery, great beef and seafood. One main N/S road, head east and you will high into the Andes, head west and you go to the ocean. Make sure to go to Valparaiso
Colombia is very inexpensive, B&B was $20 including breakfast. Colombians are some of the most friendly people you will ever meet. The hills are incredibly steep in Colombia, no wonder Quintana is such a great cyclist, in fact Colombia has some of the best cyclists in the world. They love cycling there.
Internet:
Chile, very good in the bigger cities and smaller villages.
Colombia: I found it decent but not great.
I have a friend who has done what you are proposing. He biked across the U.S., around Thailand, then rode from the NW of U.S. to S.A. through Mexico, Central America and S. A. He stopped a few times (Panama, Nicaragua and Colombia) for extended stays (months) and also took classes in Spanish. He speaks Spanish very well these days. He did not like the roadside trash in many areas, I agree as it can be overwhelming at times. He seldom camped as the rentals were so cheap. Unfortunately he did get mugged/robbed in Colombia. I think it was near Santa Marta. I did not care for Santa Marta either, much preferred Cartagena. Apparently he rode alone in an area people told him not to ride. He is fine but it left a bad impression on him. I am not trying to scare you but do your homework on the different areas you plan to ride, most are great but some are sketchy.