I agree that the boat sounds to be in rough shape. I wouldn’t put a lot of money into it. That said, you already have the boat, so why not try to use it?
For sailing on a lake, the wings probably aren’t really necessary. If one of the wings is damaged, just sail without them (at least until you find a replacement). Same goes for the sail- if it’s usable, then use it.
The damaged deck can probaby be repaired at least temporarily by injecting epoxy and/or adding reinforcing bulkheads. Epoxy injection is very easy. Adding bulkheads would be a more advanced project.
http://www.w1dm.com/Projects/HOTLINE/09-07-08%20HOTLINE.pdfFrame stiffness (or lack thereof) is probably due to either the corner castings being loose (very common) or the ends of the crossbars being cracked. You will need to take the boat apart to inspect. Disassembly is a matter of simply taking out 4 bolts (get yourself a 1/4 T-handle hex wrench).
sm