I live in Brasil and though Im opinionated, I will give you some ideas.
As for Brasil, the weather here is a toss up. CLoser to december means rain. July is colder but little to no rain (23C). Rio is a fantastic place but is very very dangerous. Stay near the beaches and the attractions unless being robbed at knifepoint is your cup of tea.
A cheap hotel throughout Brasil is IBIS. There are several around the country for about 40 dollars a night for 2. Some cost more than others but every major beach city has them. There is NO reason to go to Sao Paulo as it is not a tourist city and rains all the bloody time (i live here). Floranopolis,SC and Maceio(AL) are great places to go and are safe. Practically any beach north of Joao Pessoa you cant go wrong. If you like meat, Brasil is the place. Churrascaris, especially in the south are plentiful and can be quite cheap. The more expensive, more kinds of meat and salmon. In Sao Paulo, you can eat for about 5 dollars. The best part is that it is ALL YOU CAN EAT. Picanha and Fradinha are great meats. People say that Argentna ha the best meat. No friggin way!!!! Also u mustnt miss Foz as the brasil side has the best vistas of the waterfall ( if you remmeber the movie Moonraker(007)...)
Argentina is great too. Buenos Aires is fantastic and also Mendoza. I find Cordoba is terrible and so expensive but the lakes district there is cool but my opinion is best to aviod. The weather is best in December, not JUly as it could easily snow (it snowed in BA last year on me).
Lastly, I noticed you didnt say about going to Venezuela or Panama. I loved Venezuela as I went to Margarita ISland. Though Venezuela is a bit pricey but the black market is 5,50 compared to the bank rate of 2,15. Can chnage money almost anywhere. The beaches on Margarita will blow your mind. And its very laid-back there. If you fly from LA, Miami or NY to a place in South AMerica, fly Copa airlines. The best part is that the plane must land in Panama City and for free. Of all the countries in the world I have been to, Panama ranks #2 on my list behind Switzerland. SO so so so cheap and so beautiful. You can see monkeys, Toucans and some incredible beaches within a few hours driving. And car rentals are soooooooooo cheap with Thrifty. Panama will be one place you wont spend alot of money!!!! If you have any questions, Ill try to answer you the best I can
[ Edit: Edited on 17-Feb-2009, at 13:08 by way2goeh ]