1977 Cadillac Coupe Deville stored in a garage

1977 Cadillac Coupe Deville V8 mileage: 12,180. That's right. 12,180 tender miles! This vehicle had one owner, my wife's father. Almost never driven. It was stored in a garage for around 20 years! The interior and exterior are in mint condition (perfect!) No dings, perfect original paint, just extremely dirty. I had it towed from the CA desert in Yucca Valley to my mechanic. The engine has a large 4bl carb. Tires are all rotted/flat. The under carriage all appears to be mint also, as does the engine compartment. In your collective opinion, is this car worth cleaning up, installing new tires, and going through the trouble and expense of getting it to run properly. As of today, I don't know if it will run. I'll know that later in the week. What precautions and actions should be taken to safely attempt to get the motor to operate? I guess the fuel system could be all "gummed up" and the valves stuck shut and some internal parts rusted. If I can get it to run, what are the pitfalls of running it on unleaded fuel as I think it needs leaded fuel? If it all is worth the effort, what would an approximate value of the finished "project car" be?