Yes, it is possible to speak English fluently if you are not a native English speaker.

Yes, is it possible to teach English as a non-native English speaker.

Many Japanese ESL learners can speak quite fluently after learning English conversation in an eikaiwa (private English conversation school) for a number of years. They spend a lot of time during the week studying conversational English over a number of years.

For a Brazilian whose native language is Portuguese I would say you have an easier road ahead of you because your native language uses the same alphabet and has similar sounds. You should be able to speak more fluently and do it much sooner than a Japanese ESL learner. Focus on speaking to a native speaker often (4 hours a week or more) so that you can pick up the nuances like natural pauses, word stress, intonations, and just plain old pronunciations. Watching American movies with a lot of dialog doesn't help as much as speaking to a native English speaker because you need feedback.

Here in Japan, the trend is moving towards hiring non-native ESL teachers to teach English and this trend is growing rapidly. So even if you are not a perfect speaker, there is a good chance of getting hired as a teacher here in Japan as long as you're not Japanese.

Also for your information the TOEIC test in Japan will be moving towards having international accents in the recordings so that students can hear accents from all over the world. This means that opportunity opens up for you in a big way.

I hope this give you some positive insight in how to realize your dreams.

Best Regards,
Prodigy Creator
http://prodigycreator.com/e_index.html