I would not put in the head. I would just do the projects that need to be done to get the boat in the water. Get the topsides and the rest of the hull in a coat of paint, get the rig up, make sure she doesn't leak. Plop her in the water and go sailing. You will probably be in somewhere between $3,000 and $10,000 at this point. Then start worrying about little projects, like fixing up the interior, installing conveniences, rewiring the electrical, bedding stuff.
Fixing the core will be a gargantuan project, but if you love the boat, it can be done. Remember to always bed your hardware correctly, and make sure that all of that (plywood?) core is well sealed under a layer of GFRP or at the very least epoxy.