In general, South America is not well set up for travelers with disabilities, but the more modernized Cono Sur (Southern Cone; a collective term for Argentina, Chile, Uruguay and parts of Brazil and Paraguay) countries are more accommodating – notably Chile, Argentina and the bigger cities of Brazil. For city travel, Santiago is the most accessible city in South America. Things are slowly improving in some places, particularly in Ecuador, which now has a president who uses a wheelchair.
Unfortunately, cheap local lodgings probably won't be well equipped to deal with physically challenged travelers; air travel will be more feasible than most local buses (although this isn't impossible); and well-developed tourist attractions will be more accessible than off-the-beaten-track destinations. Start your research with the following websites:
Lonely Planet (http://lptravel.to/AccessibleTravel) Free Accessible Travel guides.
Emerging Horizons (www.emerginghorizons.com) Features well-written articles and regular columns full of handy advice.
Go Wheel the World (http://gowheeltheworld.com) Chile-based outfit that leads excellent adventure tours. Check the website for inspiring documentaries about traveling off the beaten path.
Mobility International (www.miusa.org) This US-based outfit advises travelers with disabilities and runs educational-exchange programs overseas.
Royal Association for Disability and Rehabilitation (www.disabilityrightsuk.org) Good resource for travelers from the UK.
Society for Accessible Travel & Hospitality (www.sath.org) Good, general travel information; based in the USA.