How far gone is too far gone?

I feel your pain, i bought a duster when " i was in the ether" as car Sales guys say. Looked Good from 15 Ft. Both quarters trashed, gallons of bondo , rockers rotted etc. I spent the next 3 yrs all Over the US Looking for a 100% rust free 70 Dart that hadno sheetmetal replaced and no bondo no wrecks. I got lucky and had
great timing i bought it at the height of the recrssion from a guy who was trying to keep his business afloat. i found it in Nevada in the desert perfectly preserved with fresh paint. The /6 smoked and burnt 32. Qts of oil in the 3000 miles to fl. Not a problem i had a fresh 360 magnum to install. Price $3500
If you have plenty of time and you are keeping the car, go for it. However from your description of the car i would say that after a Complete resto you will have more in the car than its worth and will be probably able to buy a Similar turn key one, maybe not as a pristine for less.
At 63 this is my laster complete every, nut, screw,washer, rebuild. The next one will be turn key.