The main facelift differences are, widescreen sat nav display, LED rear lights, angel eye front lights, newer design steering wheel. Facelift came in around 09/2000. Generally anything upto & including W reg pre facelift, X reg onwards facelift. I think the piston ring design was changed from 02/2000 to help with oil consumption.

The only running cost difference between the E46 M3 & E39 M5 is petrol ! Servicing & parts are about the same.