I'm thinking of buying a classic car?
the lemans isn't much of a collectable unless it has something unique about it. If it were the GTO it would be worth looking at, especially the 389 with tri-power.
If the mustang is clean it's a better choice. Look for rust in the interior floor, the trunk, inside the engine compartment especially around the radiator mounts for rust, check the frame for straightness. Ask if the engine and trans are original, if they've been rebuilt (ask for receipts).
take a magnet and walk around the car. Check all the corners to see if the magnet sticks, all of the lower quarter panels and the bottom of the doors. If it doesn't they've been filled, if it does they're steel (and that's a good thing).