In Pattaya/Chon Buri you get the letter of residency the day after you apply or the same day if you manage to smooth talk the immigration. All you need is your passport, copies of passport, two passport photos and some sort of document showing your address from the place you are staying at like a business card or bill. This is a very easy process in Pattaya/Chon Buri at least. If you buy a second hand bike with a plate it will still take about a month to transfer the ownership, at least in Pattaya/Chon Buri. In theory, if you land in the morning, check into a guesthouse asap, run down to immigration with a business card from the guest house, your passport and a big friendly smile, you could buy a bike (new or used) the same day as you land.