One of the most popular initial suspension mods for the E9x car is to replace the front lower contorl arms with pieces from the E9x M3. The M3 parts have stiffer and better bushings plus they are slightly longer helping you achieve a little more negative camber. In most peoples mind this is a much better mod than simply replacing the bushings.
The rear subframe bushings are soft and are typically replaced with M3 bushings but this is fairly labor intensive involving dropping the subframe 6 inches or so to gain clearance for the bearing press. Probably not neccesary if you're not going to upgrade the shocks and springs.
I wouldn't replace the rear swaybar unless you were doing a complete sport suspension install. Do the springs and shocks before the swaybars; they make the biggest difference.