Great choice--Brazil and Argentina!
Tour company--we used www.brol.com for a Brazil trip back in 2010. We chose it after scouring the internet and then sending a potential, custom itinerary to 3 or 4 companies. Kind of like an RFP or an interview. Then we chose brol.com which is based in Miami and did a lot of the negotiating over the phone. I'm sure there are other equally good companies.
Time--avoid the World Cup this June/July unless that's your goal. The same with Carnaval next year. Summer and high season is December through February. Shoulder season is good. Our North America summer could be quite cool in some parts of the two countries.
Places--Rio is great. The tour company had eco-lodge options along the Amazon. Also consider Salvador, Iguazu Falls, Buenos Aires, and many more smaller, less well known locations.
Other--Consider an airline pass. Brazil is a big country and you'll probably fly between places you visit.
Send a PM if you want more info. (We lived in Brazil 7 years.)