@TimS
To put some things to rest post the vin up to the sequencial number. Also check gtsregistry to see if it was ever on there.
I saw a few gts on line for that amount. Assuming the added modern electronics were added without butchering the wiring harness all that can go back to stock.
What are your plans for the car? Keeper, investment, flipper?
Keeper - what does the price matter
Investment - roll the dice, 65 mustangs used to command a fortune for beatup ones, today not so much.
Flipper - buy low sell high (caveat emptor)