Got one as a track car with 98K on it. I paid £4K about 17 months ago. I would hold out for a lower mileage car if possible.
There are plenty to choose from so I would take your time. The rusting issue is the biggest problem and have a good look to see if it has just been tarted up like mine had. After that it is the sub frame / chassis problem. The 330 shouldn't be as badly affected at the M3 due to the power difference. I love mine, drives like a dream but it has had all the bushes replaced recently.
I would just look for regular servcing and see if the brake fluid has been done. My calipers bleed nipples were seized solid and in the end I had to get new calipers all round.
Other than that, 60 - 70 laps of Oulton Park and she hasn't put a foot wrong. Engine is bomb proof, the N54 will go for miles.
H.