Anything from the last 10 years is good, really. You have a fair bit of riding experience and have the pace, so I imagine a 600/1000 would be a good fit, plus they are easier to get parts for. 600 is more about holding corner speed, a 1000 is more off a square it off and get on the gas. Slower turning, but more power. For the tracks here, realistically there isn't much difference in lap times at the smaller tracks at GBM this year. Note the 600 pro times vs the 1000cc times. Only mosport GP track, and shannonville pro track really benefit from the extra power. I like the extra displacement, but you do go through more tires.
You are used to a large, heavy bike, the 600's are typically quite agressive posture and small. 1000's have a little more leniency. Tim's KTM is probably one of the most spacious. If you want to have a sit on my CBR to see what the ergos are like, feel free to swing by
I expect you will be quick fairly fast, so look for something with aftermarket (Ohlins, K-Tech, Penske, etc) suspension bits already on it. They are pricey if you have to put them on yourself. A steering damper is a good idea. $5k is a good pricepoint for something not completely used/abused. Last thing you need is to start diagnosing issues or spending maintenance dollars or dealing with someones electrical mess. A few more bucks up front might save you some down the road.
Track fanatics on facebook has some options,
the wera forum is good too, but most are US based
http://forums.13x.com/index.php?forums/bikes-for-sale.48/