Ace; Hate to use my vehicle as an example, but go to Specific Car Photo Forum and go to "3639 Restoration Progress" and you will see those areas that are subject to rust and the process to address those areas. Photos of what others are doing in the restoration process are also in the same section, which will give you an idea of places to look for rust.
Also, in the section in which you posted, look at Sticky #2 for a number of items when purchasing.
As the previous post states, the angle iron welded to the inner fender support is an indication the owner was attempting a "band-aide" fix, which is in the area of rust. There are a number of inner panels (rocker panels, inside the frame and support tubing) that are hidden rust areas.
Good luck in your evaluation. Welcome to the forum and the assistance it provides.