I'm also 5' tall (I may tower over you by 1/2 inch). I like an even shorter top tube than you at 485mm on my road bike. I bought one of Terry's Valkyrie cross bikes and have been happy with it. I'm not sure they still sell them. The frame was actually made at Richard Schwinn's Waterford Precision and they will build a custom frame.
There's a huge list of cross bicycle builders over at BikeForums on the 'cross forum. And more info in this forum as well.
Good luck with your search. I'd hoped to find a used bike but never did.