You need to get under the car and look at the rear subframe mounting points. There are 4 main spots where cracks develop - the two rear mounts and the two front mounts where the subframe bolts to the floor pan.
Look for any paint cracking, rust bubbling, or visible separation between the subframe mount and the floor. Use a flashlight and mirror.
Budget 1500-2500 EUR for a proper reinforcement job with plates and braces.
EVERY E46 M3 needs subframe reinforcement at this point. Its not a matter of if, its when. Even if it looks clean now, the metal is fatigued.
I'd negotiate the price down by at least 2000 and plan to do it immediately after purchase. Its the #1 thing to address on any E46 M3.
We do these regularly. The best method is to drop the subframe completely, inspect all mounting points, weld reinforcement plates, and reinstall with new bushings.
Cost at our shop is around 2200 EUR including new subframe bushings. Some shops just weld plates from underneath without dropping the subframe - cheaper but not as thorough.
Thanks everyone. Going to look at the car this weekend with a flashlight and mirror. Will definitely factor in the reinforcement cost. The seller is asking 22k so if the subframe looks sketchy ill offer 19-20k.
