Mate - before anyone else says this - let me be the first.
You want to bring your bike up to todays bikes.....and you want more power or more look ?
Take a tip - trade it in on a mid-90's ZX6R. That will give you the power and a pretty current look.
But.........if you really want to pour money into a 23-yo bike, then you need to start with the basics...
A set of 17" wheels with decent rubber. Expect to pay about a grand for a set of wheels with good rubber. These may come with disc rotors, they may not. If they need rotors and calipers and a m/c - expect to pay another $500 (min) for all this.
Replace the crappy front suspension with some late-model forks. To buy the forks and set them up properly, expect to pay between $400-$600 for the forks, and another $500 to set them up properly with the right springs, valving and damping. You will then need a custom set of triple clamps (or modified stock clamps, done by an engineering workshop) - custom triples are about $1000, a set of modified OEM clamps will be about $300.
A new rear shock will be just about mandatory in a stock bike that is 23 years old. A good Wilburs shock will cost you around $1500. A good Ohlins will be more. Don't forget to replace all the suspension bearings as well....a couple of hundred here...
To replace all the bodywork, and give it a 'power look' (wtf?) - then a set of race-glass for a ZX6 will cost you around a grand. That is just the cost of supply. You will need to modify the glass to suit, and also make up all your own brackets.
There is a short list, it already runs to about what, $5000+ ??
And we have not even started on engine mods yet, to get the 'power' bit happening.....without even starting to look at costs of parts or labour - bank on at least $2000 as a starting point.
Honestly - a GPz600R in good condition, will be worth far more. If I were in your shoes, I would just do a restoration of what already have ( which will cost enough anyway) - and be happy with it.
You could easily spend >$8000 on doing this bike up to be as good as a current model bike - which it never will be anyway, no matter how much money you throw at it.
