I am in the middle of doing a clutch replacement on a '00 M5 I just picked up last week. As has been said, there is a ton of great info here on clutch replacements. Autozone is mentioned as a good place to get the Luk clutch and flywheel with a lifetime warranty, however Autozone is currently showing both parts to be a vendor backorder with no idea on when they will get more in stock.
After talking with the people at
www.dialaclutch.com I ordered the OEM Luk clutch kit from them for $256 shipped-I would have bought the flywheel from them too, but they were out of stock. If you want a great deal on a brand new Luk flywheel, see my post in the forsale forum here:
http://www.m5board.com/vbulletin/showthread.php?t=110948
I am not the seller, but the guy is very nice and responds to questions quickly. And you can't beat $405 for a brand new flywheel!
Best of luck with your replacement-will you be doing it yourself or taking it to a mechanic/dealer?