A 1986 300D wagon? You say "gray market" so I assume you mean this car was shipped from Europe by it's owner, as MB did not offer a diesel wagon to the US until 1987 / 300TD.

Although I have a 603, the 602's are in the same family of diesel engines and are very reliable. Head gaskets maybe an issue here but not sure if head cracks were an issue as was with the 603. Someone here I'm sure can share more info on the 602.