First and foremost, if the timing belt, water pump, and theromstat have not been changed do that ASAP. The replacement on the TB is 7-years and/or 100k. If it breaks the engine suicides.
The Pro-ams did not have subwoofers. They had two-tone seats (I have them in my Special, as does Squilly and I think 1-2 others), window trim (just a Pro-am golf logo), Pro-am floormats, and special interior trim (it was solid silver, compared to the real wood/fake wood, or the Special carbon fiber). Just be aware, the Pro-am items rank as some of the rarest (and thus hardest) to find.
There's a independent factory subwoofer add-on called the Infinity Basslink that will work in our cars.
The 300Ms come pretty loaded as is, and you have a 2002 which was one of the best years (2001 being the other, options-wise). As you were thinking, you could easily start adding some of the Special parts, or do your own thing entirely with the interior or exterior.
I'd start looking at Specials and see if there's anything you'd want on your car (perhaps the HID headlights?), and that can act as a basis. If you'd like, check out my 04 signature and see if there's anything you like with the interior that could spawn some ideas (on the last page I added recent photos and the years for the parts I used).
For the exterior you could snag some new rims - Special or aftermarket. Many do 20s; the SRT LX wheels (aside from AWD) will do the task quite well here. Then of course you have tint, spoilers, dual exhaust (the Special one or something else entirely), and of course, anything with the grille. You can also add the Special lower cladding if you like it.
Performance-wise there's really not much worth noting to add individually. Things like exhaust and intake are some, but they add minimal HP. If you want very firm shifts, look into a 2002 Special PCM (03-04 Specials had a more relaxed shifting pattern).
Last edited by Vilgent; 07-01-2012 at 11:11 PM.