The last body style was made from 2000 - 2003(no M5 in 2004).
It is my understanding that there where several changes between the 2000 and the 2001 model. After that, they where the same.
"Angel eye" headlights started on the '01' model.
Steering wheel is a little different/nicer starting in '01'.
GPS monitor is larger and slightly upgraded on the '01'.
Lastly, I'd read a service bulletin that early 2000's had a problem with some plastic/nylon part within the clutch mechanism. BMW then redesigned the part. I don't think it involved a recall to replace the part, but I imagine it is something that would be done at the first clutch replacement if it had not caused a problem prior to that.
There are probably some other subtle changes that I'm not aware of. Hopefully other m5 owners will chime in with additional knowledge.
Are you currently looking for an M5? What else do you want to know?