I'm on Samui with non o retirement visa. I've had recent experience with registering (on Samui) three vehicles.
1. Big bike purchased & registered in Bangkok under Thai friend's name (as I was originally visa exempt short stay visitor) and shipped to Samui - transferred after a couple of years to my name and registered on Samui. Was advised by traffic dept. in Lipa Noi that I needed to go to Surat Thani provincial head office and requires two visits (application request/test and new plate collection). Instead I paid an agent at entrance to traffic dept. in Lipa Noi ThB 7,000 to complete all paperwork.
2. Car purchased from owner in Nakhon Thammarat - again Samui traffic dept. advised me to go to Surat Thani to complete transfer process, as it transpired that the car was registered by the owner in Bangkok. Instead I paid the same agent ThB 7,000 and waited two weeks to complete everything on Samui.
3. Scooter purchased from local shop on Samui. All paperwork handled by dealer (red plate, white plate, etc.) and included in purchase price. No need to visit traffic dept.
My personal advice is buy local on Samui (new or secondhand) and if buying second hand ensure it's already registered to a Samui address. All you need to complete the registration is TM 30, residence certificate from immigration, house book, passport and a few passport type photos. And, if second hand privately then a visit to Lipa Noi traffic department.
It seems anything purchased/registered outside of Samui requires either a trip to Surat Thani provincial head traffic dept. or use of an agent to facilitate the transfer. Travelling to Surat Thani, staying five days, etc. is going to be more expensive than using the agent!