Hi Yozzer
here's some info from my own site on Koh Lanta that might help:
MV Flying Seahorse is a good value liveaboard that departs from Koh Lanta -- they do 2 day and 4 day trips around the Southern Thailand sites (Hin Daeng/Hin Muang, Koh Haa etc) and 5 day trips to the Similan Islands with some of the Southern Thailand sites too. Plenty of drift dive opportunities.
MV Jazz is a good choice for Burma trips - Burma is not a cheap destination, but the Jazz run the lowest priced liveaboards as far as I know.
I was on the Flying Seahorse last December and on the MV Jazz in Burma this January -- had a great time on both. Burma is fascinating diving but it's quite hit and miss, so be prepared for the possibility of strong currents and murky viz at times.
Re Nitrox - I think the Flying Seahorse is providing it from next season but not sure. Jazz only provides Nitrox on request at 200 Baht a tank I think.
Hope this helps.
Best
Chris