Learning how to look at the details has been an EXPENSIVE education for me. Hidden rust issues turn into major problems in a hurry. I've bought a "few" project cars and trucks over the years that didn't look bad but turned into, unwanted, rotisserie restorations.
I have a '66 Chevelle out for paint now that was such a case...
We're not going to talk about the "slightly rusty" '67 GTO that I bought out of ND. Turns out it had been delivered in Virginia and was a total basket case. Why I even fixed it is still a mystery. I drove to OK for a frame for it and saved the roof and firewall.