You're asking what to look for for a good rebuilding project.
If it's a rare or long OOP kit, completeness is key, now that Modelhaus is no longer available. Make sure you've got the bumpers, grille, hood, and any other vital parts that aren't available.
As others have said, the glass is important. The worst is glass that's been glued in heavily around the edges and it's melted the A-pillars. This is almost impossible to repair. If you can get the original glass out, you're usually in pretty good shape.
Body trim is important if you want to restore to factory stock. If you want to do a custom or race car, the trim's not as vital.
I never care about the chassis or engine. You can almost always find something better to use from a more recent kit.