Carrameow,
If I remember right, aren't you the one that has worked on the Mazda rotary and the dreaded Volvo diesel?
If you can keep the Volvo diesel going you can do anything!
From a person without engine rebuilding ability{me}, it sounds like you either need to find a donor engine, or another car and use this one for parts. I only say that as you mentioned rust, sagging seats, 325K! Why not just find your next MB, and get your 325K out of it?
I also commend you on being one of the most polite forum members whenever I see your posts. You're OK!