I agree with Larry on this question. Take into consideration
the cosmetic & mechanical condition, service history,
milage, etc. and pick the best one. If all these are equal,
go for lower milage. If milage is equal, pick your favorite
color

I would toss a slight preference towards the '84 over the
'85. The '85 was a one year, California only version. Still,
99% of the parts & functions are the same as every other
SD out there, but that last 1% of parts will be more expensive
and harder to find than for other SDs. Also, it will be more
difficult to find a technician that really understands the '85 than
the '84, given that the '85 is (presumably) more
complex and the tech has likely had less experience with it.

These types of changes exist elsewhere in the MB line as well.
I drove an '84 300D for a number of years, and developed a
thorough understanding of its systems and operation. While
shopping for a wagon, I looked at an '85TD. The underhood
plumbing was completely different than my '84 - I didn't
recognize *anything*. And I'm pretty sure is wasn't a
California car either.