Yes, a TEFL is a minimum. I have a CELTA, which without sounding like an arse, is much better and a better qualification. It cost me £1200 for the course at the Brasshouse.
I stand firm on the American English thing. It's not necessary IMO. It's a real bug bear of mine. Just teach normal English and if any questions are thrown up explain the difference and offer the 2 choices.