With that many miles, the car is going to need A LOT of work. I love the early fastbacks, but any old Mustang basically drives like crap after a certain number of miles. The car will need to be completely stripped down, suspension and steering rebuilt, new driveline parts, etc. I've also had bad surprises with other people's engine rebuilds. Sometimes it's not much more than a can of Krylon and some Chinese chrome valve covers.
Things like water pumps, radiators, window regulators, rubber trim and seals, wiring harness connectors, headlamp switches and ignition switches will make you crazy unless you replace them. BTDT!
If you love the car, it's worth it. Just be prepared to find a lot of crap under the surface. The price seems to be fair market though IMHO.
"Son, I thought by now you would have run out of stupid."