I do one track day/HPDE every month and use my M3, which is also my DD. I feel like this car is the best available mix of both. The primary track mods are wheels/tires, brake pads, high temp fluid, and stainless steel lines. Though, I'm leaning towards a Recaro pole position and harness.
The big worry for these cars on track is the brakes and the fact that the power steering reservoir leaks all over the engine bay. While rldzhao has started a thread about the steering issue, I've yet to fine a long term fix.
2003 CSL, SG and Michelin PSS
2008 AW/Blk 6MT, Akrapovic Evolution, Evosport Pulley, ESS Tune, Michelin PSS/Hoosier, other track stuff SOLD