Oooo! Ya pressed the wrong button. Most modern techs cringe at the word "Tune-up".
That term originally refered to the art of adjusting a piece of equiptment for optimum performance. It still does amongst the those who add chips, exhaust, and engine mods. But! There is nothing to tune about your car with the exception of the alignment.
Every item adjustable to tuners twenty years ago is tuned multiple times a second by the cars engine management system.
As to what maintenance your car's engine would need; I would suggest plugs, and filters. Plugs are simple on most MBs and a 300 is no exception. The only problem is getting the plug wires off. They just pull off (yuk-yuk). We use special pliers that grab the metal connector at the base. Even then many connectors get broken. They also are easily replacable.
Unfortunately proper maintenance involves a competent technician assesssing many items that the new systems are managing. Cap and rotor used to be routine; now they are both expensive and take some labor. Evaluating ignition using an engine analyzer keeps problems away and avoids shotgunning these parts.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician