I agree with everything being said here. I would love to sell you my M3, but honestly his old e46 is EVERYTHING you want. It won't be knocking people's socks off in auto-x, but will be plenty of fun there, and when you're ready to go to the track it'll be the best decision you've ever made.
I swore that car was going to be mine someday and it burns me that I just can't justify it. If I didn't have a 2nd child on the way and a need to tow my FD to the track, that M3 would be mine already and I would have gladly paid the $21k he was asking for it.