What year is the car? The price seems high to me. I've seen 2006 M5s with 15k miles selling for $75. Anything that I've seen around $80k has around 5 - 8k miles.
Run a few searches online on eBayMotors.com, Cars.com, Autotrader.com, just to see what M5s are selling for around your area.
Regarding the S5 and the 650.. well, those cars are completely different from the M5, and even from themselves. Of the three, the closest as far as type of car and drivetrain configuration to your SC430 would be the....
... wait a second! You turbocharged a leased car? How come you first said that you bought it?