Sounds like an awesome car, you did great finding those wheels so quickly. Driving on with a temp plate shouldn't be too bad, I did it for 60 days with the Corvette being the dealer I purchased it from was in Florida.
In regards to noise I thought the M5 had the fake exhaust noise through the speakers, from what I read online it looks like you can code those off which is maybe what the prior owner did
I would think the exhaust outside the car has to have incredible sound, I know the M550 sounds pretty bad ass, the M5 must be even better.