You can register a new bike which you've bought at a local dealer for around 800 THB, that being a Honda Wave or a CBR 150 or a Boss or Phantom cruiser - they're no bigger than 200cc. There's the Platinum 250cc twin but you want to stay away from that one.
A used bike with 250 or 400 or more cc with no book is mostly imported in parts to avoid the high import tax, then illegally assembled to a roadworthy bike. To get this bike legal is complicated, grey-area and expensive. LivingLOS at thaivisa is pretty much right on, read his post again.
Don't know how to tell you this without sounding snobbish but you should ask your questions on the thaivisa board or elsewhere. This is a website for active riders who post their trip reports.