Definately go for the M5, or you'll wish you had everytime you see (or read about) one. I would get a 2001+ if able but if not, a low mileage well kept 2000 is still a great car.
I agree with the above rec regrading m5board.com.
2016 F80 M3 Mineral Gray/Golden Brown (ED 6/3/16
2002 E39 M5 Sterling Gray/Caramel
2007 E86 Z4M Coupe Silver Gray/Black
1999 Mercedes ML320 (hauler...)