looks in much better shape than the prindle I had, and trailer looks ok too. Sails are better also. Look really closely at the hulls and the hull/deck joints to see if there is a gross defect and pound on the hulls lightly and you will be able to tell if there is significant rot. A gallon an hour is a lot (in my opinion) and would be coming through a place you would notice. I just got done building up the centerboard wells on my 6.0. I was going to do the vaccuum test but then just figured if water is getting in, then it will get out also- so I filled the hull up with a little water and found it leaking out of the centerboard well at a crack. Might be worthwhile to try this with the prindle you are looking at. Now that there are pictures I thiink 900$ sounds pretty good (given the sails and general appearance).

If the hulls seem intact (no rot, weakness) then it is most likely a fitting which is easily dealt with- if you want to deal with it.