It isn't unusual for crud to grow on the exterior of a trailer after a few years in the elements. I would bet that the trailer you are looking at would look dramatically different after some elbow grease cleaning it up. 70's trailers also had a clear coat on the panels, and it is typical for this coating to be pealed/vaporized in the upper surfaces of the trailer. Again, the fix isn't to replace the panels, it is typically just to strip all of the clear coat. If the panels are dented, perforated, or dramatically corroded (looks like white powdery stuff), then replacement may be in order. I would not pay anywhere near $5000 for a trailer that needs panel replacement.