"Muito Obrigado." That's how you have to say "thank you" when you come to Brazil. Well, yes my friend it is important that you learn at least a little portuguese before you come here. You ask if the political situation here is stable. It is stable enough, so far. In spite of being a "revolutionary" in the past, our President Lula is very neo-liberal today. And I think that is good for you but, unfortunetely, horrible to the people.
Brazilians think Americans are too proud for their own good.
It is important not to act as if you are coming from the land of milk and honey straight to the land of alligators and bananas. We don't like this sort of treatment. If you behave properly, according to the laws of our country, not taking your clothes off on the beach, as some English Marines did, when they came to these shores, you definitively won't have a problem. Our people are very nice and polite. They are even a little bit naive, in my opinion, as far as americans are concerned. Some of us, still tend to see you as superior. But many others, specially among businessmen, have a different view on the subject.
Summarizing, if you get aquainted with our laws, and obey them, be nice to the people, don't be arrogant and see us as equals compared to you, even if it is not true at all, than everything "is gonna be all right"! Of course, surprises always occur, who knows the day of tomorrow?
I hope I have helped you in anyway, sorry to be so sincere about this matter.
Welcome to Brazil, and enjoy yourself.