Rust, rust, and rust. Trim bits are a bit of a faff to rummage for but there's nothing that really can't be found if you look for it, mechanical bits aren't really too much of a problem either. It's the bodywork that's the real pain. Front wings are NLA as you've found out, but so are quite a few other panels. The club does a good line in repair bits, but unless you can weld and paint that's an expensive way to go.
Typical rust spots are basically anywhere that could get stone chips around the front of the car, the bottom of the front wings where the sill starts, the sills, the rear arches, the boot floor, the vent panel behind the rear screen, the A-post just where the door frame meets the front windscreen, the front floor pans, the bottom of the doors, any bit of the car that has a vinyl covering (the vinyl on the pillars is rigid so doesn't "blow out" like the roof, but if you push on it, it will still make a crunchy noise if it's rusty), the chassis legs and the headlamp mounting panels. Aside from that, there's no problem with rust.
In all seriousness you'd need to buy a complete dog to get rust in all of those places (I did). But I would go with
1)Body Condition
2)Fixtures and fittings
3)Does the engine work
As the order in which to consider when buying. As a result you probably should be looking at things that don't have an MOT so that you don't miss cars that have been sat in garages for a while without being used. Of course it all depends on how much you are looking to spend.