I just bought a KZ 550 a few month back, basically the same bike. The GPZ has dual brakes, fairing, a little higher top end, etc...
LOVE IT! It will smoke my 550 honda in a second, but does not look as good or sound as good! IMO
Yes I bought it when it was snowing here in MI.
Here are some FYI's your after...
Very reliable motors. They were used as mail route bike in Europe and some had over 250,000 KM before they retired them (read somewhere on kawasaki forum). The carbs are a pain in the a$$ to take off compared to my honda. Main jets are nearly impossible to find for the gpz model. cam chain tensioner spring likely will have to be replaced, too weak from factory. Valve adjustments are a real PITA! you have to remove the valve covers and cams if you have to swap out shims to adj the clearance. There is a little trick with a zip tie to avoid having to take the cam chain off the sprockets.
kzrider.com
and
www.gpz550.com/scripts/gpz_forum_2005/default.asptwo valuable forums for lots of info. neither are nearly as good as this one, takes a while to get responses and the search functions are really pretty useless, but there is some good info there.
If you like the bike, go for it.