I used to have a modded 98' GTP. Very great cars in every aspect.
Easy to make tons of power yet still get great gas mileage and driveability!
First off Congrats on the purchase. The E-bay link doesn't work anymore.. What are the specs on the car?
Best starting mods would CAI, aftermarket PCM (Intense, Overkill or DHP are common ones, I personally had a Digital Horse Power one) and U-Bend Delete. The U-Bend is right after the Catylic Converter and really restricts the exhaust flow. All you need to do is get that U pipe cut out and a straight chunk welded in its place. The reason for the U is so they could mount the o2 sensor straight up but you can mount it sideways into a straight pipe or get an o2 simulator. (That is what I used and never had any issues)
The PCM makes such a huge difference in throttle response and everything.
After those are done then I would go with a 3.4" Pulley. Any smaller and you will get some KR. Take a look around on some of the Grand Prix Forums, the guys are very helpful and there are endless mods and possibilities!
This is a site I am a member on still:
http://www.wcgpc.org/
Good luck and I am looking forward to some pics.
Any questions feel free to PM me!