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.
I negotiate by offering a year paid in advance with no deposit. Landlords can rarely resist that much cash in one hit. Be prepared though to do any minor repairs yourself ( I can and do ) You won't see the landlord for a year. At the end of the lease you can renew or walk away.