www.m5board.com is the best to use.
I have an 01 M5. The year I would avoid is '00 since it doesn't have the widescreen nav system. I had the dreaded carbon build-up issue and at the time it was a $9k repair bill (I got BMW to goodwill almost all of it - I should have just reported them to the EPA since it is an emissions issue that should be covered under warranty). Now you can just pay $1k to PowerChip and have it ignore the secondary air pump that throws the CEL.
(1) Sensors will have to be replaced every 70k or so.
(2) The engines do occasionally eat oil 1q per 1000 miles is considered normal.
(3) The repair bills are that of a 75k car.. not a 19k used car.
(4) Look for rust around the gas cap
(5) If you are going to install performance parts be prepared to pay a whole lot.. they make Porsche parts look cheap.
(6) Some people eat through clutches quickly.. and it is expensive.. 2k for new clutch + flywheel
(7) Get a PPI
(8) Drive as many as possible
I love mine and used it as my daily driver and will track it occasionally. It is a great q-ship and is a blast to drive, if you can do some of your own labor it'll make owning the car a lot cheaper.
If you want to know anything else you can PM me or just ask it here and I'll also post up anything else I can think of