By far the biggest headache is the automatic climate control system.
Failures of either the amplifier or the servo are common.  Either can
be easily replaced, the AMP goes for about $120, the servo $600.  The
servo most commonly fails by developing a coolant leak, which isn't a
good thing in a unit with 4 cooling hoses, about 10 vacuum lines, and
about another 10 electrical connections all hooked to it.  If you do
need to replace the servo, Performance Analysisn in Tenn. sells
remanufactured ones that replace the lower crack prone plastic housing
with an aluminum one.  And if it fails, they will fix it for $100 for
life.

Check cars you are looking at by cycling the climate control from full
heat to full cooling and make sure everything is working. IT is also a
good idea to do this periodically about once a month, as this runs the
motor, mechanism, feedback potentiometer, etc in the servo through
it's full range.

The servo is under the hood, near the firewall, passenger side.  Make
sure there is no sign of coolant leaking.

Other things to look for are rust, which could be a big problem
depending on where the car was driven.  Pull the floor mats, front and
rear and inspect for rust there.  I just pulled some parts from one
headed to the junk heap, and in front there was no floor left, you
could see the road.  I suspect this one rusted from the inside out, as
that area is common for water pooling from blocked AC drains, window
leaks, etc.  And check the vacuum system which works the door locks,
trunk lock, fuel lock, etc. to make sure it works.  The system is
complex and can develop leaks.

If you get one in good condition, these are tough strong cars, some
would say among the best MB has made.  I had an accident recently,
where I hit a 3 year old chevrolet.  I barely felt the collision, the
Chevy was totalled, I drove the 300SD home.