hey,
i think the oil filter cap should be on the driver's side of the engine bay between the black plastic intake manifold and the firewall, look for a black plastic cap.
i only use engine oils that have the MB spec. 229.1 or 229.3. as far viscosity goes ... that depends on the weather conditions in which the vehicle will be operating. check your owners manual, there is a scale in there . i run synthetic mobil1 5w-40 .
as far as how much oil. i forgot

but i do know that its either 6 or 7 quarts.
i personally drain at the plug, after taking the car for a spin, to ensure that the old oil is nice and warm/hot.
as for the fuel filter, the Haynes manual says to replace it every 36,000 miles
DIY W124 Gasoline Fuel Filter R & R
enjoy the new ride.