One of the other dark side forums has an
FAQ covering teaching in thailand, visa's and work permits.
As for work permits, I believe that non imm O marrage visa allows you to apply for a work permit. the FAQ states that a degree is required for a WP as a teacher, but as with everything in thailand, manditary requirements are not always mandatory when the boss isn't about, people working out in the sticks and for well connected agencies have managed to get one.
So it looks like if you do go for the job, the school director needs to make it very clear in his supporting documentation that hes been looking very hard for a very long time and that you were the best he could get.
I have no knowledge of the tax laws, but I know quite a few salaried civil servants earning 20-35k/month, non of them pay much tax at all.... significantly less than a months salary per year