I would replace that door. Wouldn't bother fixing or trying to fix that.
My 1981 needs paint now too. For the minor sections I'll have some rattle cans made for touch-up in single stage. For a whole door, I'd get the paint gun out of moth balls, etc...
If I was interested, I'd look at the price of the truck, then take the price of a new door, paint, labor etc off the price and see what the owner says etc...
Truck: 1981 F-150 Explorer / Engine: 300-6 California MCU Feedback System / Trans: T-18 - 4 speed / 2.75 Ford Rear Axle Open Diff.