Mine came to the states in '78. Rust Not nearly as extensive as yours sounds. Just some of the thinner exposed sheet metal was affected. The ends of the inner fender supports are rusty(common with coupes, exposed to road debris)-I am fabricating new ones.
Here's a cab conversion 250SE coupe on BAT that's commanding a high price. It's had obvious rust repair
https://bringatrailer.com/listing/1966-mercedes-benz-250se-cabriolet-conversion/