Remember that cash is king. If it looks good and you don't have to spend $500+ in tires, battery, cables, brakes, carb cleaning, bulbs, switches, etc., then you just make a lowball offer and leave your phone #, after no-one paying what he wants, he calls back to accept your offer, or you call him back in a week to ask if he wants to sell it now. You aren't really missing-out on anything.
I once owned a KZ550 Kawi ( also KZ750 & 3 KZ1000's) and had a lot of fun on that bike. The frame was a bit small for best comfort, but the smaller bikes are much more 'user-friendly' to learn to ride on. Four cyl bikes are much faster than twins, most of the time (hey we ain't talkin' Ducatis here!) and might sustain someone's interest and needs for a longer period of time. The XV535 is a good bike to learn on.