do they vary on milage?

they're all pretty much the same.
any difference would have to be attributed to jetting changes.

Differences between 1996 & 2003 are "bold new graphics", colour of tank/seat/bags. Go for the cleanest year you can find.
Check all the usual things - how often the oil's been changed, air filter, bearings greased, top end replaced.
Kawasaki recommends tranny oil being replaced every 600 miles/1000k. I think this is too long, so replace mine about every 200k. I like to be able to recognise that it was oil once, not black sludge. (previous owner must have stuck to the book on that, as first oil change I wondered what he'd put in it lol).
If it's missing an airbox lid, don't stress about that, it's a common modification. Check out the top post in this forum about what to do when you get a new bike, lots of helpful tips in Canadian Dave's pages, along with a timeline & all sorts of other info.