What about a WRX STi? Or even an Evo? I know.. ricer
Maybe they are all over where you live (then again, if you live in Cali, M3s are all over the place too). Audi S4/RS4? MB Class AMG (C32?) could also be comparables. How about the previous generation M5? Thats larger
Ok.. So, you'd like to be able to haul, on occasion, more than one person.
What other things in your life would you need more space for? Why not just be the one who "gets a ride" instead of giving the rides?
My dad has a previous gen (E36) M3 sedan. It is a fantastic car. I would argue it to be equal or better than the Boxster (even an S) from an ease to drive and performance value. I would say even better steering feel than my Boxster had. E36 cars, even "excellent" ones, can be had for under 20K, with most in the $12-14K range. They are a great value and also worth considering. Heck, you could save some coin by going a bit older, and have a little nest egg for that 996/997 down the line.
I don't think either car is a bad one (E46 or E36 or Box S). A friend of mine recently traded his 1995 993 Coupe for an E46 M3 because he wanted room for his little kids. Loves the M3, but misses the 993 everyday.