You need the certificate from immigration. They will need to see proof of address. Take the docs you mentioned. You also need a doctors certificate that your fit to drive. Many clinics issue or hospital depending where you are. Process will prob take 30 min to 1hr at immigration.
After confirmation letter he had to get THAT letter certified. It seems only applicable to Brits. He is in Hua Hin and he didn't want to go to BKK so had a lawyer do it. She sent someone to BKK
I am retired here also and have both thai motorcycle and car. As I have a car I stopped riding a bike. Its quite dangerous here and I personally know experienced riders who have seriously injured themselves. You buy a bike for 20k+. Thats a lot of tuk tuk or 10baht buses. Much safer.