* Would a TEFL course be useful to have and enhance my chances of finding work? Would the fact that I am a native speaker also add to this?

A TEFL course would DEFINITELY help. Just because you are a native English speaker doesn't necessarily mean that you can teach

* I am currently trying to teach myself Portuguese though I expect to learn much more easily on arriving in Brazil; do I need a much higher level than my current 'basic' in order to teach?

Unless you are teaching beginners you don't need to be able to speak Portuguese to teach English. TEFL qualifications allow you to teach anywhere in the world. Many of my (qualified) friends have taught in a number of countries without knowing the host language. You'll be surprised how well students can learn English without needing to speak Portuguese. In fact when I was studying for my Trinity Cert one of the modules was learning a foreign language - I had  Swedish lessons (as an absolute beginner and after three lessons was able to tell the time, say my name, where I was from, what I did for a living and count with NO english spoken at all

* I'm planning on starting in Salvador de Bahia; any particular reason anyone knows of that I would find it more difficult to find work here as a teacher?

There are already many VERY good English teachers in Salvador. Most good schools (such as ACBEU, Cultura Inglesa, and Pan American)  would almost certainly not consider you without TEFL. My wife used to be a DOS (Director of Studies) at ACBEU and used to have to turn away native english speakers due to their lack of experience (a TEFL course gives you this - although I'd be very careful about a 'weekend' course. I did mine over 8 weeks part-time, and it involved practical teaching experience as well as the theory)

Sorry to sound pessimistic, but it is better to be realistic about things and hence better prepared to fulfill your dreams.