The exhaust is a must IMHO. Can't stand the sewing machine sound. Ness Honkers and Atom Bomb are both reasonably priced. I de-baffled mine and am quite pleased with the sound (only $100 too) Lots of others to choose from.
LLoydz timing wheel, air filter, and PCV, made this thing run like it should! Modest investment makes a HUGE difference in performance and ride-ability (less heat, smoother, faster revving, more fun:-)
Stereo was also a great return for the money also. Rockford amp and speakers (lots to choose from, Polks are popular, use the search). Less than $500 and much better sound quality, not just "louder".
Yes I have done all these, and I have not had one moment of regret. I made the mistake with my last bike (HD) to rush out and buy every little bolt-on that caught my eye. I regretted it later. It's sort of a good thing they don't make that many accessories for the XC
Take your time, research, and have fun!
BTW, we'll need pics....