Im a true Duster guy at heart and the (realistic) prices have dropped substantially, yet everybody still wants a arm/leg for them. Theres a clear reason why nobody wants to pay 20k for it so why should you?? Sounds like you have a decent corral of cars so show him the money of what you 'feel' its worth to add to your stable and if no deals are made, then return to the bank and make a deposit.....that always feels good too.
Lets just 'say' you paid the 20k....
Rebuilt motor 'correctly'....$4k
Usually leads to other things and cleanup/replacement.....$2k
Time and hassle unless your paying someone else to do it.....either way 1 season minimum and possibly $???.....
All for a car that once is finished, you MIGHT get your $20k back.....but i still doubt it. Your call....depends on your Love for the car and how deep your pockets are.....just my 2cents!