I'm an e46 m3 owner and there's plenty of good pointers here.
www.m3cutters.co.uk is the place to go.
The only thing I would add is to google for BMW customer service no. They can tell you off the reg. no. whether its been through any recalls and what the main dealer service history looks like (helps tally with the service book too).
Generally I'd make sure it's had a recent insp 2 service (or budget accordingly. Vanos, front brakes and rear subframe mounting points on the body are the main key points to scrutinise. If its done 60k + miles the broken rear springs and worn out rear trailing arm bushes are prob not far away either. No big dramas to replace either (& springs go alot at any age it seems),, but car will feel vague at best or be lethal in the wet.
Lastly if it's your daily, budget for winter tires come October, or take the Bus if it snows!
I love mine btw, don't let these things put you off! Try and get the best spec you can (eg digital tv, sat Nav etc) & enjoy.
Cheers
Iain