Air Asia do Bangkok to Surat Thani too, usually V cheap. Buy a bus and ferry ticket when you arrive in Surat Thani. We stayed in Mae Nam which is quiet but I personally preferred it that way.
I've been to SE Asia quite a lot in low season, I prefer it at that time, it's quieter and cheaper. Samui and Pha Ngan don't get affected too much by the weather and even in the west it's usually only an occasionally thunderstorm followed by good weather.