Do you have a Mustang club locally? If so, you could probably get someone from there to check it out for you. The biggest problem with '66 models, is the stuff you CAN'T see. You need to be able to check frame rails (especially front), floor pans, cowling, and trunk floor side panels. None of these problems are show stoppers, except rotten frame rails. You should also check around the bottom of the battery for rotten radiator support, and corroded inner fender panels.
Get a parts catalog and check prices for that minor stuff. If you've never owned a classic before, you might be shocked by how quickly the $$$ adds up for a little chrome thing here, a new knob there. I'm not trying to talk you out of buying the car, but be honest with yourself about what you're willing to spend on the total project, and just how much it will actually do what you have planned.
BTW... I have a great '66 coupe...but I spent $35k to get it where I want it, and I did all the work except paint & headliner myself. Have fun!