I am no expert by any means, they will be along shortly.
i have an 82 car the main places for rust seem to be:
a) inner wings at top (easy to spot)
b) battery tray in boot
c) the back of the front wheel arches as they join the sill and floor pan, (there seems to be a built-in dirt trap to cause corrosion which can rot this whole corner out and if you cant weld it yourself will be expensive to have sorted i guess)
d) the sills themselves
e) the rear hatch door and around the rear lights
and last but not least, just when i thought it was all done
f) behind the rear wheels arches, the rear chassis rail to the bumper mounts, this seems to be another dirt trap built-in, and you can only see it if you properly get under the car, as its mostly hidden by the rear quarter bodywork.
I have also been warned of them rotting from inside the roof because of poor drainage if they have sunroofs and the front scuttle/bulkhead.
and the wheel arches too.(thankfully mine was good here at least)
Apart from that make sure you drive it for a good run to check the gears, engine and brakes, the usual for any car ...
oh and check the jacking points too
Best of luck mate.