I agree with Greg on the vents. They are great.
The early 240s are more rust prone than the later ones, but they are all FAR better than Australian and Japanese cars of the same age.
If a particular 240 isn’t rusty now (cars that are full of bog with shiny paint recently applied still counts as rusty ;) ), then it is highly unlikely to become a problem in the next decade.
The main places to check are:
Under the front edge of the bonnet;
Around front and rear windscreens;
In the door jambs, particularly the rear;
Sill panels (especially if there is rust in either of the previous two places);
Around the boot opening/under the tail lights;
wagon rear side windows (especially inside);
Lower rear quarters, especially if there is rust in the previous two places.
But yeah... rust has claimed far fewer 240s than Kingswoods, Commodores, Falcons, Sigmas etc etc.