Buying a project car with frame rust - where is the actual line between fixable and not worth it

Looking at a 1987 pickup that needs full restoration. The body is clean enough, but the frame has surface rust running along both rails. Not flaking apart, but definitely more than cosmetic. Seller is pricing it like a driver, not a project.

The question that keeps nagging me is where people actually draw the line. Structural frame rust is a completely different conversation from floor pan rust or rocker panels, and that distinction gets collapsed in a lot of buying advice. Pitting, scaling, and active flaking are three different situations, but the guidance people give rarely specifies which they mean.

This truck would see real stress on those rails: road trips and occasional trail use. A frame that looks borderline in photos but sounds solid when you tap it can still be hiding wall thickness loss you won't catch without a bore gauge, or at minimum a serious handson inspection underneath.

Curious what inspection steps people actually use before pulling the trigger on something like this. A magnet tells you almost nothing useful here. Has anyone had a shop do ultrasonic thickness testing before buying, and is it even worth the cost compared to just walking away?