Good cars if you find the right one. I tracked one down a few years back for a friend prior to our annual Le Mans trip (and yes it did struggle to keep up with my fire breathing Alfa

).
There are lits of tired and thrashed examples around, but if you know the car it helps. A few things to look for...
Rust! They are 20 years old now and rust can be rampant. They didn't get as much protection as regular E30s. Tricky areas are behind those plastic sill covers. Both inner and outers can rot at both ends. Boot floors can rust out, the bottoms of the A pillars behind the splash panels can also rust- open the door wide and try and lift it up- rot at the bottom can cause flex.
Being a performance car, it will have been driven enthusiastically. Look for accident damage and also stress cracks between inner wing and bulkhead, and between rear inner wing and boot floor. Finally whilst looking at the body, check for water leaks. They can smell musty inside when they have been leaking. Front and rear screens, and boot rubber will be the culprit. If it has leaked then there will be rot under the seam sealer between floors and sills. Check e erything twice as its £££££ to sort properly.
Oily bits. Engine. Most get thrashed. Check for big bills. You are looking at £6000 in parts alone to top and tail that engine. Listen for excessive valvetrain noise- they are not the quietest engines due to steep camshaft ramp angles, but should not be excessive. A little smoke when cold is ok even on a rebuilt motor.
Clutches are weak due to a relatively small size and modern grippy rubber can only exagerate the problem. Gearbox is tough and will do high milage as long as shifts are not rushed when cold. Diff can last for ever if its notbeen abused but driveshafts can fail. Rear subframe bushes and wheelbearings are often overlooked and can make an otherwise good car feel tired. Brakes are weak compared to modern fare and should be kept in tip top condition or upgraded if you want to get the most out of it.
Interiors can look scruffy and don't wear well. Electrics can be bad too on tired cars. Trip computor and check panel often pack up or go haywire, and electric motors for windows often are slow. Whilst on the subject of elecronics, get an extra electric fan fitted infront of the radiator. The radiators are too small when driving too hard or abroad and having an extra fan that cuts in around 75 deg won't go amiss.
All common sense stuff really, but worth thinking about. A good example can be a joy to own and give years of trouble free motoring. A bad one will eat you out of house and home, and when you come to sell you will struggle because most M3 buyers are pretty savvy and know the pitfalls involved.
Cheers
Si