First thing to look for is rust.
1, front cowl at each corner. There are drains in the recess for the front windshield. They get block by sealer during r&r of the glass and start to rust.
2, The jacking pads under the car .
3, the lower body seams are damaged because people use them as jack pads.
4, the rear panel at the license plate lights, and the license plate lower mounting screws is rust prone. The tail lights tend to cause rust on the panel as well.
5, the composite body cladding tends to hide rust that starts with the fasteners .
6, In the last ten years, the reputation of the car has gotten the attention of many people, and as a result there have been not a few body shops that have taken advantage of the rarity of the car and the avidness of owners, to turn out some real sheds. Avoid restored cars unless you can prove the providence of the shop doing the work.
7, the VIN# can be found on all body panels, and stamped on the cowl strip to verify originality and rule out accident damage.
8, Accident damage can be well repaired, but even so, rust does not sleep(imo) so it is best to avoid car that have been hit too hard.
9, With the exception of the hood , the body panels are bespoke to the M3.
1, Check behind the fuse box for cracks in the firewall the result from track use with R compound rubber, but no bracing between the strut towers.
2, Check the front frame rails from the engine bay. Look for straight seams and a uniform appearance to the rails.
1, If the car is near or just past 100k miles and there is no documentation, then it is a wise economy to have the rod bearings replaced.
If this is not done, one can anticipate around $5k for a total rebuild in 30k miles .
2, If there is a clatter on start up when the nose of the car is uphill, then the hydraulic chain tensioner requires replacement.
3, the engine requires a valve clearance adjustment at regular intervals.
Otherwise it is a robust engine when in good fettle.
4, The engines are commonly chipped to raise the redline to 7500-7800 rpm, so think twice about rod bearings on such examples.
The engine components command a hefty premium due to their bespoke nature and low volume.
The geometry of the suspension is different to that of the other E30.
1, the front strut/steering knuckle are only available to this car and are very expensive. Otherwise the suspension can be bodged using regular E30 parts.
2, The car uses 5 lug wheels as opposed to 4 lug on the others.
3, front wheel bearings are larger than the other E30s.
1, Rotors and front calipers are bespoke items.
2, Rear calipers accept pads from the E36 range.
3, ABS sensors are the same as the other E30 but the ABS ECU is of a different calibration.