You can purchase the entire M3 kit for ~$2k from Tischer (getbmwparts.com). While you have the rear subframe off for the rear sway, it's worth replacing the subframe bushings as well. Also, you'll need to align the car to M3 specs after installing the parts.
HP Autowerks (hpashop.com) might be your best shot at the best possible track suspension setup, with both M3 and even stiffer aftermarket parts. I'd just drop Harold (username - HP Autowerks) a line and let him make the recommendations. He's one of the most knowledgeable guys here.