I guess if you even contemplated buying something that old, you have some skills to work on it. The two biggest things will be the manifold (also, if it has been run too long like that to cause any failures)and the water intrusion. The rest is just typical maintenance.
I would do a pressure check on the engine and inspect the plugs on the cracked manifold bank. A quick prod or two on the ceiling will let you know what kind of shape things are like underneath the ceiling panel.
Good luck on negotiations. If you have the ability to fix on it, I think it will be a nice rig. It's the kind of setup that some hunters dream about because of the 4 x 4.