It depends on what you're looking for. Bear in mind that you're talking about cars now that are pushing forty years old.
As Dave said, rust is the main problem. 200B sedans will rust first at the bottom of the doors and guards, then the bonnet and bootlid and surrounding boot area and rear beaver panel. This then causes rust the rear quarter panels to rust. I've also seen some sedans get rusty underneath the fuel filler cap. The same goes for a SSS. Wagons will rust under the cargo area windows and the roof gutters as well as the other aforementioned places. Depending on how handy you are with a welder I would suggest staying away from a car that has excessive rust in the rear beaver panel.
I would expect that most will need an engine rebuild by now if they haven't already had one. In my experience back in the day a blown head gasket was extremely common. If they have had any abuse then budget for anther differential too, they are exceptionally weak. Gearboxes tend to hold up fairly well.
Aside from all of that, general wear and tear. Bushes and suspension components will most likely be very worn by now unless they have been replaced. Which they most likely won't have been.
_________________green200b - aka Alistair
'80 200B GX - the green200b
'81 RX-7 - 12A extend port, 4.11 Torsen LSD - daily
'74 RX-2 - 13B bridgeport, 4.44 LSD - cruiser
Green200B has forgotten more about 200Bs than we will ever know.