Welcome to the forum. I am a relatively newbie here, myself. I would advise you to READ. Do a search in this forum on the topics you asked about. There must be 50 pages of posts about how to tell if a diesel is in good shape. The archives in this forum can provide you with more information about diesel Mercedes than you could imagine.
In a nutshell, Don't be overly impressed by the condition of the interior. If the seats are covered with MB Tex, they will always look good. Leather, especially if not maintained with show a lot of cracks.
Cars with known service histories are a bonus.
Realize that for the first year you may be performing a lot of maintenance that the previous owner deferred.
With the engine warm and running, remove the oil filler cap and observe the stream of exhasut gases flowing out of the cap. It should look like a tea Kettle on boil; not like an old steam locomotive at full throttle.
Look for rust.
Rear floor, and sills, and jack points.
Read the old posts.
Read the old posts
search the forum.