I recently bought a pre-owned E46 M3 with 28K (Canada) and have had not problems but lots of thrills. However, I bought mine from a BMW dealer and it was "Certified" which includes a VERY thorough check and repair of everything (it was close to new looking and performing) and a very good extended warranty among other things. I did this mainly because I was concerned how the first driver drove the car, especially during the 1200 mile "break in" period. I also checked out all the BMW service/maintenance and other work done on it (re accidents, etc.). Maybe I'm paranoid but I wouldn't buy a pre-owed BMW except from a BMW dealer and have it certified. If things go wrong it can cost you big $ and impact your resale. Also, my understanding is that the new M3 will have minimal impact on the value of the E46 M3.
Anyway, good luck with the search for the new car and hopefully you'll be drivin' in the sun soon.