I had my 81 300SD painted this last summer. It had that Burgundy color, and the clear coat peeled up in several places. Can't just buff it out with that.
Not cheap to do, but I had all the body work done (door dings) and shop painted with DuPont products. Looks very good now. Still need to get the headliner replaced. But the list just never ends with these old diesels.
Mine had an absolute minimum of rust problems, but the shop still found a spot on the front corner to mess with. If you have surface rust you might consider having it looked at my a competent body shop before treating it with anything. There are lots of products out there that will make a small problem worse.
But for now, just enjoy your ride. Get the car running and stable before taking on that one.