The top 3 for track-bound Zx10's of mine:
1) Tires (DOT Race tires... cheaper than new bodywork
2) Rearsets. I grind the crap out of the stock ones... and rearsets are cheaper than my boots too.
3) Front brake upgrade(s)... calipers and lines in my case do the job. The stockers don't cut it for me.
...beyond that it comes more down to taste/track. Gearing is a good idea, as well as removing anything you don't want to replace in the even of a crash. Things like exhaust and such are nice, but not at all necessary. I have a weakness for quickshifters, so that requires a PC3 as well.
If you're going track-only with the vehicle, get some 3 piece bodywork and chop/unbolt/lose everything that the bike doesn't need to run. Low mass is it's own reward, heheh.
Handling? Hrm, I like a little extra ride height in the back, and I've played with lowering the front as well. Those two, combined with aggressive tires and gearing will mike the bike pretty exciting on the power.... especially over bumps. If/when you get to that point you might find that a damper upgrade of some sort is in the cards. I think you'll be amazed at the handling difference you get just out of the DOT race tires; it really is night-and-day. When you're good enough to need high dollar suspension you'll know it.... 'till then you can tweak and dial the stockers up to a pretty fast pace.
What and where do you plan on using the bike for? Each track has its strong/weak points that people can recommend mods for. :) Setting up a pure track bike is a lot easier than compromisng one for street use.