i suppose the rust problem depended upon where the car lived and how it was cared for. mine lived in texas.
the 1976 450sel was my first sonderklasse benz. it was a great road car. the seats were only manually adjustable, but that was ok, i was its only driver. originally, it failed to provide enough legroom, but benz offered a kit that allowed another 1-1.5 inches of travel.
it did have power windows. and the climate control was ok, but eventually the uv and the heat in gulf coast texas destroyed all the vaccuum lines. but that was at the 230,000 mile marker.
car never experienced any major mechanical problems. absolutely bullet-proof. and it was driven hard all its life and routinely did 15mpg. in 1986, when the climate control system went south, i replaced it with the 560sel. which i consider the zenith of car design and construction. and as a lagniappe, added the 560sec to the stable some months later. a fabulous pair - masterpieces of the 20th century.