I would never buy my phone here. I use iPhone and apply pay. Thais can’t use Apple Pay. If I buy a phone here, I can’t use Apple Pay. Thus, I buy in the US. But your phone there and when you get here, get a Thai SIM card for it. Keep your old phone with your US number in case you need it. Been working for me for years. I don’t notice the $60 dollars I spend to keep the US number.
I highly recommend you get a 60 day tourist visa which you can extend for an additional 30 days when going for a retirement visa. I also, highly recommend you do not try to come on a one way ticket without having an exit ticket. I recommend getting a ticket to someplace like Singapore or Vietnam for your exit ticket. Either get one you can cancel or eat the $50-$75. When I presented my one way ticket the airline was going to deny my boarding until I showed them my exit ticket. DON’T chance it.