69 mustang

Sounds like it needs a lot of work still to be complete. I'd say the car is worth $5,000-$7,000 but then I am always known to low ball the value of vintage muscle cars. Getting the body work and paint done is going to cost the most, in California I bet you'd be looking at $5,000 to $10,000 minimum.

The issue is if the motor is still in good shape along with the transmission. Sitting still for a few years is not a good thing if the car was not prepared properly for storage.

It's worth quite a bit actually. The engine having never been run has pro's and cons. The seals may have dried up, and the cam would most certainly be bone dry by now. But the plus is everything is there. a simple seal kit would fix it. I would take it mostly apart for new seals and lube EVERYTHING. with the engine, trany, brakes, exhaust, wheels, radiator, etc you're saving alot of cost. The fact that its an older gentleman who has had it for 30 years is a bonus as well. In reality it would probably go for 8-10,000. If the parts are good enough it could go as high as 15-20, but if they are that good its a decent deal. AND a great starting block, considering most people buy a 6 cylinder car and in the process of converting it buy everything you listed anyways.
Start the bidding low, if you have intentions to fix it and sell it don't mention this to him. Do the opposite in fact, make him believe you'll keep it forever (which i hope wouldn't be a lie). Most people of the older generation like to see and help one of the younger generation get into this hobby and will cut you a deal for that fact alone. good luck and let us know how it turns out.