Buy on condition, mileage and service history. I have an early '99 and I've upgraded most parts to to facelift spec. The cosmetic stuff is quite asy to do ie lights, nav and screen. I haven't done the steering wheel as of yet but I belive this is fairly easy to do if a tad expensive. Mechanicly the Vanos is slightly differant and the oil consumtion may or may not be less on 2001+ car. For the record mine drinks oil like an irishman on st patricks day but there are quite a few 2001+ cars do the same and pre facelift cars dont drink any so I wouldn't use this as a basis for buying.
Find a colour/interior combo you like and if its pre 2001 the you can pick and choose what upgrades you like. Thats the key to M5 happiness. Trust me, I did it the hard way and bought a dog and have spent god knows how much making it shine again! But I did probably save it from an early grave so thats something I guess!
Good luck