There is almost no such thing with a 300D. There are no spark plugs, so there for no distributor, no plug wires...actually there is no ignition system period. There are glowplugs, but they can last a lot longer than sparkplugs, and usually do. I suppose an oilchange, and a bottle of Redline diesel fuel catalyst, along with the occasional air filter, would qualify as a tune up. Now on a W108...there is the entire ignition system, plus there is the valve adjustment every 15k, then there is the chassis lube every 5k (or less), and god forbid you need new doorseals...they are $100 a door.
Price advice? It depends on how bad you want it.
Besides, you really ought to take both of them for a test drive...not just each one, but if you can find others like it, drive them too. then you'll be better able to make comparisons as to what you are getting for your dollars.
Got any pics of the car?
-Larry