iMessage, international texting, and domestic texting

Hello, I'm currently using the iPhone 5 in Brazil. I bought my iPhone 5, factory unlocked in the United States about 3 weeks ago. As of right now, I do not have a SIM card for a Brazilian carrier but I will soon get one. Right now, I am sending my iMessages through my email.

Once I buy my SIM card here, will it it still be free if I leave the "Caller ID" option as my email?

If I need to text a Brazilian friend, I will have to turn on my phone number within the Caller ID options. And if someone iMessages me from the United States, will I be charged with internaitonal texting?

Is it possible to keep using my email solely for United States numbers and my actual number here in Brazil for Brazilian numbers?

Thank you very much.