+1 on above: rust is the big issue.
Not sure a seller would be too happy if you start attacking their car with a screwdriver! Magnet is useful tho.
I wouldn't be too surprised to see signs of welding - ok as long as it's a proper repair and not capping some rotten tin.
As long as it's not making any horrible noises, smells or smoke, there's only so much you can find out on a viewing - the journey of discovery starts when you get it home!
I'd fully expect to have to attend to some consumables on a car of this age - brakes, suspension, steering, fluids...
As with any old car, it helps massively if you can do your own maintenance - there's always something that needs attention and labour bills can soon mount up.
With any newly acquired old car I would allocate a few days to get her up in the air, wheels off, carpets out, and a thorough clean. Find out where the mud traps are and be prepared to treat any rusty spots immediately.
There are some good products out there - I use Fertan for surface rust, Dinitrol for injecting into cavities.
A dry garage is essential, as is a good trolley jack, axle stands, ramps, breaker bar, decent sockets, selection of wire brushes for drill & angle grinder, safety glasses and gloves, and a multimeter.
Fingers crossed she's a good 'un, and enjoy!