In all honesty, go to m3forum.net and read the stickies at the top of the E46 section.
Listen closely for VANOS problems. Anything that sounds like marbles is bad. Get the subframe checked. You really need to get a PPI done on it. M3's can get expensive quick, especially if anything happens to the VANOS. A "good deal" can turn into a money pit.
Mileage isn't an issue, but maintenance is. That maintenance goes way beyond oil changes. It's due for a service 2. Has that been done? If not you're looking at dropping $$$$ right off the bat.