I agree just offer $500 you have nothing to loose but I would think on the open market and if it's not to rusty that car would at a guess get $4000. The blue interior is worth quite a bit on it's own, you have an original radio and an un-cracked ash tray. All these things add up to a lot of $$$ if you have to go out and find them or pay over the top for ebay stuff. For you intrest I have spent far in excess of $25,000 restoring a Z so I would hope to be in a reasonable position to tell you the costs.
Anyway $25,000 would be with other people doing alot of the work but you'll have to do all the leg work to get the parts together its also heavily dependant on how much metal needs replacing. You'll need a lot of new old stock parts if you want it mint? Of course if you want it to look average then just paint the outside, por 15 the engine bay and try to get that old rusty engine to work just don't show anyone under the bonnet
CB