Firstly stay on a Kwiks-saki which it seems you will. Next thing is physical size such as seat Height, weight, riding position. Most likely scenerio is it falling over

(lets be ready for that) while your going slow so dont foget to push them around with motor turned off as part of the test rides. Three point turns etc.
The Naked Bike Z750 would be way cheaper to fix after it rolls off the side stand at the servo & it has the lowest seat height of the current crop, not by much though, the ZZR600 is the lowest seat height by heaps & will cost most to fix n be hardest to pick up as its the heaviest by a fair bit.
My advice would be an 02 ZX6R Lowest seat ht of the zx6rs n best looking by a mile, new enough to get a good one old enough to not cost too much.
Enjoy the test rides n have a ball on your new scoot,
Cheers Brett