If your going to spend this kind of money on a car, then I'd try sticking with a BMW dealer or a one owner private party. Reason is, you know very little about the background of the car and you need a reasonable amount of assurance that your getting a good M3. It's very likely this car, on a used car lot, was bought at auction and auction cars are usually there because they were a selling problem for someone.
At the very least, I suggest you take the vin to a BMW dealer to see if he can track some of the history of the car. When you are satisfied there, then do some research on the background of the dealer. Make sure you want to do business with him (or her).
Good luck with it.