a thing a lot of people overlook, especially here in the northeast is the frame rails inside the engine bay. They rot out fast and are expensive to fix if they even can be. About half the mustangs in MA that i have seen have rotted our frame rails inside the engine bay.
i bought a mustang from new jersey and the framerails are the worst i have ever seen so yes .keep particular attention underneath the strut towers they are notorious for rotting away right there