I always look for rust in the rockers. I don't care if it is a W110, 111, 108, 108, 114 or 115 the problem is:

Rain falls on the automobile. It runs down the window through cracked rubber seals. It collects in the bottom of the doors (which is why you see rust along the bottom of the doors) where it should run out of drain holes in the bottom of the door. These holes get clogged with dirt and you get a collection of water in the door bottom, hence the rust.

If the water runs out the holes like it should then it lands on top of the rocker. This is protected by a rubber sill unless you have a 300SEL and then it is stainless steel.

If the rubber gets cracked the water just stands on the top of the rocker, after it has seeped through the cracks in the sill. Then you get get rust in the rockers.

So look first at the lower outside window seals. If they are cracked and/or gone then look hard at the bottom of the doors and then the rockers. Tap them with a small rubber hammer and you can hear the difference between solid metal that that which has thinned out.

And that rust on the doors? That's not surface rust. It is rust that started on the inside and has eaten its' way through.

Look also in the spare tire wells. Water collects there and, just like the doors, will start rusting in the wells and then start eating away at the outside body of the car.

And none of this info is the last word on the subject. It is, like a lot of info on these cars, a general guide. Before you dive into one of these then study, study, study.

And keep this thread going. Ask questions. No one here is shy about admitting mistakes they have made in buying these cars.