Thanks for the good pictures and advice. I use a paint thickness guage when I can inspect the car. Unfortunately, I have to rely on pictures most of the time.
The latest candidate is 600 miles away. Here are some picture of the underside that the owner sent. The car has been undercoated and I have cropped the pictures to highlight the worst areas. Does this look like surface rust? The floor pans look great from up top.
The rest of the underside looks like the clean part of the picture.
Should I run away or does this look like a manageable repair.