I think a 15.5 frame is what you need. The big test is if you can straddle the top tube and have a couple inches of "fudge factor."

As for price, it might be a bit steep. If the bike is in primo shape, $650. can be a fair price. $500. sounds more reasonable for a now 4 year old model. It looks like a quality ride with good basic components. That is just how I see it.