Newbie needing answers!

What are some areas on the body I need to look for as far as rust? I'm a welder by trade but never done body work.

The frame rails, the trunk, the lower quarter panels and the cowl to start. Since it looks like the car was recently primered black, there may be some filler underneath it. If you can and the seller allows take a pen magnet and go up and down the body, if filler is in excess the magnet won't stick.