I've had a e46 m3 for a couple of years now. Brilliant car, average about 22mpg in mine but does less when kicking its head in. Ins can be cheap (£450 for. Me) on a classic policy but they limit miles. Brakes can be expensive and deffo need upgrading if you do a track day. Check for tearing subframe, they will all do it it's just a matter of when. If you buy one under ten years old, get bmw to check the subframe as with a bit of haggling they should replace the boot floor as a goodwill gesture. Not cheap to tune, two real options csl air box and alpha N remap (expensive but sound awesome) or supercharger (expensive but can make 450 to 550bhp in different stages). I'm saving for the supercharger, 5k plus import tax from the states but should be a reliable 500bhp. Rusting arches are common, front wings are about 170+Vat a side dunno about rears. Build quality apart from that is good. On m3cutters there are three camps -
1)Purists who think all m3's should be left standard and they are the best cars ever built(twats).
2)Manual gearboxes are best.
3)SMG gearboxes are best.

I've got Smg and love it so I'm in no3 camp lol. I apreciate it can cost a bomb when it goes wrong, but fast driving and track days are brilliant fun!

Main thing is buy the best spec and condition that you can buy, apart from the harmon kardon widescreen stereo, option as it's shit and you'll have to spend a fortune upgrading speakers and head unit etc.