I'm not an expert on these cars but I do have two rusty coupes. Additionally I've examined a few drivers over the last couple of years. I'm not sure what you read, but this is what I would focus on. I have some pics in the project thread of my car that highlights some of the areas discussed. My parts car does but that is unrealistically rusty. I'll just assume that you are looking for a 'project car, with all of its inherent flaws.
Front End:
Check the bottom of the front fenders, near the passenger door. Look for signs of metal patchwork and body filler on the outside. From inside the fender, look up into the area near the windshield frame. There is an access hole up there and you can snap a pic of the inside with your phone. I refer to this area as the fender box, but that isn't the real name. Also from insid the wheel well look on the front side of the fender where the back of the headlight would be. Check for rust holes. With the hood open closely examine where the inner fender meets the outer fender. Look near the shock tower for signs of repair, filler, excess seam sealer, etc. You should be able to see the factory sound deadening, or the factory spot welds. If the whole area looks nice and smooth it has likely been reworked. Check to ensure that the firewall seal runs all the way from fender to fender. It isn't uncommon for the ends to get cut off when working on the innner fenders. Check the lower windhield frame, particularly near where the end of the passenger wiper sits. Check for filler just below the chrome trim.
Cabin:
Open the glove box and peer up at the firewall. Look for signs of rust, holes, or repair. This is the other side of the aforementioned fender box. The firewall tends to separate from the kick panel area but it will be covered with carpet. You can feel around in this area and also verify from the wheel well. Next check fuse door. I haven't seen much rust in this area even on my disaster parts car. Peer up above the fuse block and look for rust. You also have slightly better access to the same area where the firewall meets the lower A pillar, where the kick panel is. Look for separation.
Rear Seats:
Ask for permission but pop off a rear seat. Simply lift up on them. I would chose the driver's side only because I've seen more rust on that side (I have no idea why). Look at the curve of the wheel well and check for rust. Pay attention to the area where the well meets the floor. If you can, check the other side. If you look at my project car the wells are fine on that car, but the subframe pickup points have quite a bit of superficial surface rust on them. When the rear window seals go this area gets pretty wet. As a result, you could have musty/ mildewy seats and the wooden backs of the arm rests could be heavily water damaged.
Undercarriage:
Peter's video has this covered. Check out the floor pans. This is pretty obvious. If they aren't shot they could be on their way, particulary where the large metal floor plugs mate with the pan. Also check the frame rails, particularly underneath the driver/passenger feet.
Read End:
Check the spare wheel well. Check the strut tops for signs of damage, stress cracks, repairs (not necessarily a bad thing), etc. Check the forward section of the wheel wells. The outermost side of the well that isn't accessible under the seat. Look for holes forward of the rear tires.
Rockers:
They are likely covered up by trim. Give them some knocks with your fist to see if you get any falling flakes. Stick your finger in the drain holes and see what you find.
Doors:
I've seen a number of doors with rust holes near the window chrome. Check for body filler. Also examine the underside of the doors for rust.
Rear windshield frame:
Check the base of the rear windshield. Look for signs of a bad cracked seal. Look for body filler particularly near the fenders. Open the trunk and check the center of the body lip that the trunk seal rests in. Check the fender sides as well, and the lip on the tail panel.
Paint:
Look for signs of overspray. Closely examine the belt-line trim and windshield seals. Determine if the car was masked and sprayed or all trim was removed during the painting process. The paint job could be great but it is an indicator that shortcuts were taken. Shortcuts with body filler and the ones you want to avoid.
Interior:
Look for good seat cores that are solid regardless of the upholstery condition. Look for door panels that aren't warped. Look for a headliner that isn't sagging. Most importantly, the condition of the dashboard. You can get seats, carpet, headliner material, door panels, center console, glove boxes, and fuse doors if you hunt around. It is very hard to find a clean crack-free water-damage-free dashboard. The condition of the dashboard wood particularly near the A Pillars is important. Look for significant delamination. A few top layers are workable but repair outfits (some of them highly recommended here). The door wood is less of a concern but it does tend to delaminate on the ends as well.
