The S52 is a 3.2L, while the S50 is only a 3.0L, giving it a bit more torque down low. The S2000 would be my personal choice, but it is likely the harshest riding and least practical of the group. I like the Boxster, but the ones in your price range will probably only have the 2.5L engine, leaving it a bit underpowered. I like the B5 S4 quite a bit, driven one hard several times, but it is a bit on the heavy side (though I believe it is the most powerful of the group). The E36 M3 is also awesome, quite a bit lighter than the S4 and it has a silky smooth inline 6 (again try for a 1996+ with the S52).
The best combination of autocross performance and practicality is probably gonna have to be the M3. Personally, I'd say fuck being practical and get the S2000 because it is an awesome driving experience.
As far as suggestions go, you could probably find an early WRX (not STI) in that price range as well. I've also seen early 350Z's in the 11K-12K range. RX8's are renowned for their handling prowess and can be found right around your price range as well.