This is what the UK gov website says about driving here
"If you are planning to drive in Thailand, see information on driving abroad. You’ll need to carry both the 1968 version of the international driving permit (IDP) and your UK driving licence while driving. You cannot buy an IDP outside the UK, so get one before you travel.
If you live in Thailand, you can apply for a Thai driving licence using your UK driving licence."
I have had 3 of these while living here, number 1 was signed by my GP - she charges for doing it. Number 2 was signed and stamped by reception in my condo and the third I have just had signed and stamped by the piya baan in my village.
1 and 2 accepted by DWP without a problem and I assume number 3 will be as well
It took me about 15 minutes in the local hospital. I took my passport on to the reception desk they copied the details on to a form, stamped it and carved me B500
I live in Kanchanaburii and in August 22 took my UK licence and IDP, along with a health certificate and cert of residence to the local DLT office and got my Thai car and bike licences without having to take the test. I'm on a non O visa
I bought 2 Honda scooters new a couple of years back. I chose to buy new because all the second hand bikes I saw had unbelievable milage. Other factors were, seeing how the local kids thrash their bikes and how infrequently they go in for service.
If you know the seller I'd say ok for second hand bit of you don't, IMO, your safer with new