I was very hot for this same model M5 a year ago. It was going to be a 4th car that doesn't get driven very much, just when I needed 2 extra doors and needed more room then the 911. My biggest issue was that for my driving, I just couldn't justify the car. I only live 10 minutes from work and its all through residential property. I would have to take side trips to run the car and feel its 400 hp.
Chkbookmechanic is giving very good advice and it is similar to what I have heard. Just don't be fooled by the initial starting price with these cars. You are buying an bona fide supercar without any warranty. Hopefully nothing major will happen (but it does occasionally), but the minor maintenance on the car (brakes, clutch, tires, radiator) can also be pricey. I ended up buying a 2003 BMW 540i with the "M" package. It looks almost identical to the M5 (I debadged it and added the small spoiler).
Here is my old post:
http://forums.pelicanparts.com/showthread.php?t=400973&highlight=bmw+sedan