Check the air filter on the 1987 300D. It is easy to ignore in maintenance and if blocked would prevent you from getting proper air flow and might increase smoke.
It would be helpful if the EGR should happen to malfunction, because that would decrease the soot being sent back into your oily intake to make tar-sludge.
Congratulations. You've got more guts than I do by far. I've bought on EBay, but I've always seen and driven the vehicle before handing over payment.
Ken300D