Most of what you will be looking for you will find by putting the car on a lift. Rust is the big killer, Check rear wheel up rights where they connect to the frame. check the rockers, floors and sub frame sections for rust. Check under the radiator as they often leak and rust out the support.
Check the ZF trans, does it shift smooth, funny sounds, etc... Pay special attention to the trans, rebuilds start at $4,500. It is not uncommon to have electrical issues, so check every electrical component.
Back to mechanical, these cars overheat very easy. Start it, drive it then let it idle for 10 min. do the fans come on, at what temp? Now the engine is a 351 Cleveland (basic mustang engine) so don't be too concerned about minor engine issues. Water pump although a pain in the neck, is cheap to replace. because overheating is an issue, check to make sure there is no water mixed in the oil, blown head gasket. Engine issues are not usually a deal breaker, its not a Ferrari.
Minor stuff to look for are, is the steering wheel cover original, they are getting very difficult to find. front steering play, rack and pinion bushings tend to go after 30,000 miles. Are the calipers seized, etc...
I have had a bunch of Panteras and loved each one. They are great cars and the owners tend to be emotionally connected to them, as am I.
No issues with buying low, and making a few $ to support your family and giving someone a good deal at the same time.