I own an M5, and have had to replace sensors here and there. Its not as bad as all previous comments state. However, do some research on how to DIY and your could save a boat load on the repairs. Also, ensure the car has a great track record so no hidden electrical gremlins poke their head.
For your best advice, I would go to M5board.com and talk these the avid M5 enthusiasts on there. I wouldn't bother on here - no offence, but all the chaps on M5board do is eat, sleep and talk M5's
*** I am 32, with full coverage on my 2001 M5 is $113/month. as stated above, clean record and safe driving = how much you pay.....