Yes. When you transfer funds you need to ensure that you are issued with a form stating that the funds came from overseas to ensure hassle free repatriation of your monster profits. As you are long term I would recommend Seamico as they routinely deal with these expat issues. Boom.HK.com is an alternative if you are long term. Commish is 2x what you would pay with Thai broker but they will accomodate GTC orders and 'if done then' which could be an advantage if you are in another time zone. Not for active trading though, limit orders only.
There are a number of index funds offered by local mutual funds and SET 50 futures will begin trading this Friday, but frankly I would not buy the index at this juncture as there is a polarisation between a few large cap oil related stocks which are driving the index (PTT, PTTEP, TOP, SCC) and the real economy where many stocks are at cycle lows. This is EXACTLY the time to buy them. If you are in another time zone you need to buy them while they are still basing and ahead of the conventional chart breakout point as when they near this point the 'kha yai' (big hands) come in and sweep the book and take them up very fast.
Disclosure: Boots already filled