I have a 83 300SD. Actually it looks OK from the outside, but underneath it has enough rust that I really shouldn't be driving it. The chassis members that run under on both sides are terribly rusted, especially on one side where it is completely rusted through, I think in more than one place from memory.
When I try to jack the car from the left front, the frame actually bends into the door and you can't close the door when it's up, if it would even go up all the way which I doubt. One of the diagonal members that is welded to one of those side members....isn't ... the joint has completely separated from rust.
If I get around to it I'll look at the structure and analyze how weakened it is and what is still actually holding the car together! Maybe just habit..

I'm basically waiting for the car to sort of split apart underneath and then it will be ready for burial - I suppose so will I if it happens on the highway at 70+ mph. Otoh it might keep going for another 10 years or until the engine claps out - it's only done about 200K.