My advice if you are thinking of long term living is not to choose an Island as Island life can be challening and limiting at the best of times and more often than not more expensive than the mainland, As for visa speak to a reputable agent or Thai Immigration for the correct advice.
A few weeks ago I would of said get an agent but due to all the 'bullies' on here saying do it yourself I recently did my own retirement visa and saved myself 20,000 Baht , take it one step at a time and don't stress, immigration are very helpful here .... Oh and thanks 'Bullies' 🙂