IMO,
Rust and previous damage are the #1 issues. Even IF you can repair and replace sheet metal, it's a PITA. grannyknot@ is correct, any rust you see is only 20%. Many people have given up on their project when they discovered the other 80%.
Complete is #2. Kurbycar32@ sums it up pretty well. Some parts are getting expensive and if you take it apart, you'll have a better idea how it goes back together as well as having the fasteners.
Mechanical is #3. Hard to judge that if the car doesn't run or drive. Compression test on the engine is possible using the starter. Does it roll? Does it run? Do the brakes work? Wear patterns on the tires can be indicative of other problems.
It's very difficult to help you with the value of the car when we have little information to go on. Give us some more info and pics and you'll get a lot more help.