The SD is bigger, has electric seats and a dedicated electric pump for the door locks. Everything else is about the same as in a 300D.

The window motors aren't difficult to replace, but having skinny arms and two elbows per arm would make the job easier. In most cases the motor is good but the platic slider that lifts the window disintegrates. less than $10 for the part and a robust nautical vocablulary should see you through the job.

If the rear sags, new subframe bushings will improve the ride, handling and tracking. It has the same front suspension as the 300D except for a more complex rear mount for the guide rods.

The ACC system is very similar to the 300D's.

I've never had to deal with a rusty car so I don't know where to look for rust or how difficult it is to repair rust.

Sixto
91 300SD
87 300SDL
83 300SD