TW, I'd love to see that car if it's local. I've owned that exact model, (sold it about a year ago), they are really nice cars if it's a good one. The problem is that most are not good ones. Mine had the updated cylinder head, (#22 casting), most likely changed by the dealer long ago because of trap oxidiser failure leading to engine damage. It was a common problem on the OM603 engine.
I have a pretty serious MB diesel addiction. I even have an extra OM603 engine complete but I'm probably selling the short block this week. This is my 1980 wagon on eBay right now, I need to thin the herd:
Mercedes-Benz : 300-Series Base Wagon 4-Door in Mercedes-Benz | eBay Motors
I'll send you a PM, I might be interested in the car but I can definitely give you some input either way. There are many, many long threads concerning that engine on the big MB boards. I like them a lot.