Just bought a 1985 fox about 1 year ago..
here is what i looked at..
1. Rust, i live in the rust belt, so honestly if there is any rust, there is more you just don't see..
2. Leaks, make sure you bring the car up to "really warm" make sure it doesn't leak, oil pan gaskets are typical, as are rear main seals and valve cover gaskets. Intake manifolds also leak, make sure you check the oil, look for any clouding, as it could be head gasket, or could be the intake gasket leaking, they are both simple to fix on the older 5.0 engines.
3. Smoke, a little smoke under the hood on a freshly serviced car, is different than the smoke you will get from a contact leak. if it is thin, and eventually goes away, it is likely an oil change drip on the headers..
4. Doors, check the way the doors close, make sure the doors hit the hit points correctly, and don't sag, this isn't horrible to fix, but will show if the car has been beaten on. (Chassis flex).
5. Convertibles, Check the top, the top itself is about 1200, but the frame, and motor are a real pain to fix, so make sure they work correctly.
6. Convertibles, Put the top down a little, so the top is sticking straight up, and look down into the boot area, check for moisture, if it seems damp in there, chances are there is rust you don't see.
7. Carb'd cars.. The carb in the 85 and prior cars is typically replaced, if you live in an area where you don't have to go through emission controls or you historic plate the car, i would rip the original carb out, and all of the smog crap, as just a simple new carb, and no smog stuff will help the car breath much better..
8. CFI cars, you will likely want to avoid, since the original CFI cars are a nightmare to tune, and are for that matter hard to get parts for.
9. Body parts.. most of the fox parts you can get, but the doors with the mirrors that are not mid door, are not reproduced, so check these extensively for rust, otherwise it's a fiasco to find doors..
10. Paint, the paint sucked from the factory, panel gaps are terrible and things never lined up from the factory, so checking gaps is pointless..
11. Mileage, They don't have the 6th digit, so make sure you have the title, and check for stickers for oil changes, those might help to determine if the mileage is correct, or if it has flipped.
12. Low mileage cars, this isn't always a good thing, this typically means it was stored for a long period, which has likely dried all the seals, and adds to the amount of work you have ahead of yourself..