Aside from the obvious you should know that its rear suspension is
hydro-pneumatic. That means there's an engine mounted hydraulic pump,
rear leveling valve and two nitrogen chambered shock absorbers in the
rear. Not terribly expensive to fix but you should know how it works.

Its only point is to keep the car level, regardless of rear end load -
same system is installed in the 560 SEL sedan (but not the 420 SEL which
has a conventional suspension).

I have an 86 sec, have put lots of money and time into it. It is a classy
looking car and cost over 60k new. But it is a very complicated piece of
machinery, lots of things to go wrong. Some things you can work on yourself
and EBay is a gold mine of new and used parts. I had to send the speedometer
to Texas to have the odometer repaired. Bought a used cruise control
actuator on EBay, also new armrest cover and steering wheel cover.
The car has lots of power, but you will get about 14 mpg on high test.
$$$$$$$$$$