I've owned a 636 since upgrading from my starter ride years ago. After a chat with a former WERA racer his thoughts were that it was an ideal track ride because of the extra oomph. Its pretty cheap and they changed the geometry in '05 from the previous generation ('03-'04). So if you look I'd stick with the '05 or '06. I've had some troubles with my '05 brake rotors warping and the '06's are thicker. Going rate is roughly $4k. See kawiforums.com, a sister forum; it's pretty active.
In that era I found all the others to be uncomfortable for my 6', long-limbed shape. You'd be well served to sit on whatever you can find to determine your comfort level on each of the major models. My wife loved the GSXR.
An alternative I've considered are the CBR600RRs that have ABS in the recent past, which was highly rated for a while.
Dragging knee is for the track, and dragging tail is for the lot. --Kane Friesen
When you're in a car, you're watching a movie; when you're on a bike, you're in the movie. --Robert Pirsig
Identity theft is not Fun