I think the price is fair, especially with the low miles.
Honestly I was looking at 96-98 Cobras earlier aswell and found some nice deals within about 200 miles.
For what it's worth, they would probably try to trade you straight out. New Edge cars suck at holding their value.
If you were to get that car, I'd buy it and then sell your GT private party and get the most out of it that you can, which will likely be more than trade in value.
However... I'd keep the GT. Your car is pretty bad ass and your plans for it are going to make it even better, and with that Cobra you are starting fresh.