MOTS services I think are 25,000 baht for visa, 4000 baht for bank account. They will cover the 800,000 thb and sort you with a Pattaya address if you don't have one.
You can get a residency certificate within minutes. Usually if you buy from a dealer they will take you to Immigration to get it. But you can still buy a car and do the transfer later when you have the certificate ( If you are buying privately )
The exchange rate isn't too bad at the moment. I am getting 22 baht with OFX. Bring cash with you and get a bit more or use an agent will save you a heap.