I recently came back from a 4-month trip from Brazil. For the entire time I rented from Localiza (http://www.localiza.com): They have one of the best rates and they have many offices so if you need help there is always a Localiza nearby. The "big" companies (such as Avis, etc.) are usually only found in bigger cities. With respect to codes, I don't know if Avis has them for Brazil and if it even pays. Good Luck

P.S.: Unless you are in one of the bigger cities/airports, you might have a hard time finding English-speaking agencies, so brush up on your portuguese!