What is the process and document requirements for moving to Thailand for six months and retiring there later?

Okay, seeing as you're only wanting to stay here 6 months next year and not retire until 2020, lets burn that bridge when you get to it and address next year first.

Probably your easiest way to stay 6 months would be to get a 6 month METV <-multi-entry tourist visa. That visa is valid for 6 months from the date it's issued and each time you enter thailand you get stamped in for 60 days. At the end of that 60 day period, you stamp out of thailand, stamp in&out of another country, then back into thailand for another 60 day stamp. You do that for the validity of the visa itself.

Once you get closer to 2020 (which in thai time is a LONG LONG way off) then you can look into options for a long-term stay. Believe me, no one posting on here has the slightest clue what the immigration rules will be 3 years from now.

Good Luck.

When retired and want to live here: Non Immigrant O visa based on retirement, than apply for Extension of Stay at local Immigration for 1 year. You need or 800.000 Thb on Thai bank in your name, or income verified by Embassy of 65.000 Thb a month or a combination of these 2.

Option 1. Non Immigrant O visa multiple entry 1 year, gives you 90 days admitted to stay, border run for another 90 days. Option 2. Tourist Visa 60 days with Extension of Stay at local Immigration for 30 days at 1900 Thb than neighbor country for days and apply for new Tourist Visa 60 + 30. Option 3. Multiple Entry Tourist Visa, 60 days each entry plus Extention of stay 30 days.

Find a Thai girl and marry her

To retire, you have to be over 50 years old and have 800 000 baht in Thai bank. Then it is easy. Six months stay you could do it on tourist visa with couple hops in and out of Thailand

you might read thai embassy website for 6 month tourist visa and retirement visas (50 years up)