Not a big fan of the M3 huh?
Well for 20K you could get a kickass Porsche Carrera (like the old school 993 ones, i think), but yes, most parts will be a bitch.
I still say swapped RX-8, as the 3 rotor would put out more driveable torque, and it would probably be the only one of its kind in your county. Downside is, it won't
legally pass smog (at least in Cali, not sure about Michigan), since the motor was never in production in the US. Also, the swap would probly run you some $$.
S2000's are nice, reminds me of a 4 wheeled version of a motorcycle, with the high revs, low torque, and gearing to remain in the powerband (i.e. 4000 rpms in 6th at freeway speeds). It's one of the few cars where you
won't be making power at 4000 rpms. I bet it would make a great autocrosser though, and maybe you could get a hardtop for it.
Evos and STi's are awesome cars to me, but if you don't want 4 doors oh well. Funny that they are the fastest in your list though.
Honestly, I was never a fan of the boxter, I say drop that off your list.
Just hold on to your money and buy a Lotus Elise...can't ask for much more.