Cali posted some great info on Muay Thai Gyms in Thailand. I'm in Chiang Mai, Thailand right now doing a 6 month Muay Thai camp if anyone has any questions.
I started about 4 years ago and have been coming back to Thailand every year since to do 3-6 month Muay Thai camps. I still consider myself an intermediate but I've had five fights so far and have trained in Phuket, Koh Lanta, Koh Tao, Bangkok and Chiang Mai.
If you scroll back through the archives of my blog there is a lot of information on there about what to pack, when to come to thailand, what the best gym is for different goals and personality types, how much my monthly budget was and reviews of different muay thai schools. I trained at a lot of the big Muay Thai/MMA schools such as Tiger Muay Thai and Phuket Top Team, as well as some of the smaller traditional Muay Thai schools. Check out the blog, it'll help you plan your trip:
www.MyFightCamp.com
The best advice I can give you is, it'll never be the perfect time to come to Thailand to train. You'll always be lacking either Time, Money or Both. If it's something that you are truly passionate about, make it happen asap. I know that for me, I had a million excuses why I shouldn't come out for 3 months, but the minute I landed, I knew I made the right choice to follow my heart.