I have no Idea where to start

That’s a good looking car! get under it with the trunk open in daylight with some eye protection, a scraping tool and a wire bush and poke scratch anywhere you see daylight. It looks like there is still a lot of good sound sheet metal in the car. If you haven’t already, carefully around the subframe and wheel wells and around the front spring mounts. Check out the center crossmember braces closely, as that part of the car carries the suspension load of the torsion bars. Do the same up at the front structure where the k frame ties and bolts in especially at the cowl area. It’s good to double check all the load bearing box and cross structures to confirm they are all solid, some areas can hide rust working from the inside out. A slag hammer is good to find it with but be gentle and use common sense, get any bare surface rust or metal cleaned up and get some permatex extenze rust treatment or POR15 on it.