So what was the problem that they were able to correct so you can test drive the car again? I would walk away from that one and try to find another one that looks and feels perfect - mine is
My 550 has been flawless and I have not had any issues at all. It only has 21K miles on it but still . . I've owned it for over three years and I have never had a single problem with it. I suggest you buy your 550 from a one owner private party, and get the service history from the dealer the owner used to service his car.