When you buy a motorcycle, especially your first motorcycle, they not have to be shiny new. For example a used Kawasaki ZX-6R. In 2002, Kawasaki thought: 'Hang on, why does a 600 have to be 600cc?' and hogged out the cylinder bores of their old 599cc by another 37cc.
The result: noticeable more power and speed, with a midrange that was unrivaled in in its class until Triumph introduced the 675. The 2005 Kawasaki ZX-6R model has softer suspension (Showa rather than Kayaba) than the earlier 636cc ZX-6R's, which is why you should see it differently. The suspension change seems a minor tweak, but it was all it took to turn a total headbanging motorcycle into a polished all-rounder that could topple, that time, Honda's CBR. The pre-2005 Kawasaki ZX-6R's have no real faults. It's a solid motorcycle without a doubt. You want to look for small improvements like uprated suspension, maybe a steering damper, a hugger, a Scottoiler... that sort of things.
Don't be tempted to cut out the exhaust catalytic converter, it's part of the collector box and you'll shatter the power (avoid any Kawasaki ZX-6R where you can see this has been done). If you want a loud can, buy a aftermarket exhaust system, something like a Akrapovic system instead. Double bubble screens are also a good suggestion, and work very well.