I am looking to buy a 1985 Mercedes-Benz 300D with an inconsistency in the ignition shut off...with a delay of sometimes 5-7 seconds...how serious is this problem??

I would think the first thing is to check if the fuel on/off arm that the old 220d has, is completely pushing forward to full fuel shutoff. I imagine the 300d has a different mechanism, but the same problem, incomplete fuel shutoff, so it uses the fuel already in the system til it shuts down completely. Again, I have no experience with the 300d, but that sounds the problem. Other's will post, I'm sure, with 300d problems.
Ralph