Just get any AF-D lens; the D60 can't auto focus with the screw driven lenses (AF or AF-D), but can meter with them and all the camera modes are available. The Nifty Fifty (50mm f/1.8 AF-D) comes to ...
I'm not sure what you mean by limited used market, there's truckloads of used Pentax gear available. Going pro depends on what you mean. If you mean full frame, then there is no upgrade path at ...