Hello gtdave,
I don't think the dynamics will necessarily change. You seem like a competent, outgoing and fun person to be with, so I wouldn't worry too much. It takes a real man to accept help when he needs it, for example with language and social life in the beginning. It is clear this girls has strong feelings for you and they won't change overnight unless you change your character drastically.
What you can do is try to keep your independence as much as possible. Don't jump at the first opportunity to work for their company, but keep your business going - in case things don't work out as you hoped for, it will be great to still have your business.
Establish an independent social circle as soon as possible and strive to learn the language quickly, maybe you can start with a language course right away.
Make sure you feel ok leaving your life behind, but if you crave adventure, this shouldn't be too hard, just make sure you're doing it for you and not "for her". If you keep this frame of mind, I don't think she can lose respect for you, since you're not following her, but have decided to do this for yourself.
I think this could be great for you, nothing opens up the mind as much as living in another culture. And last but not least, she does feel a bit insecure about other girls, and rightfully so, judging by the stories I've heard about Brazilenas

So you still have the upper hand in being the prize. Don't overuse it though.
People who live in Rio usually fall in love with the city, it's a special place, I think you'll have an amazing time there. It may be hard to be the gringo at times, but really, the fact that you're an American there is an immediate and permanent DHV. The only trouble is, you never know if people want to be around you because of you or because of your passport, so be careful when choosing friends. On the other hand, If you're moving in the circle of rich people, you don't have anything to worry about.
Good luck and let us know what you decide!
P.S. I envy you
