Very easy....no need 90 days proof... As long as u have a valid visa, bring your passport n 1 of these, utilities/phone/credit bills, etc with your full name and TH address....book online with SG embassy, head down to Sathorn. Go in the morning, tell them u want to buy a car....afternoon u will get your residence certificate.
With residence certificate, passport, driving license, 1 suitcase of THB... walk into any car dealer, choose your car, if new car is available and no problem with insurance and paper work, u can drive out your car within a week. No need to wait for COE bidding.