If you need a manual immediately to fix a leak or a window motor, digital downloads are the modern go-to. These are typically searchable PDFs that you can use on a tablet while under the car.
The FSM is the original documentation used by Mercedes dealership technicians. For a car as over-engineered as the 300D, this is the only source that provides 100% accurate torque specs, vacuum diagrams, and hydraulic system layouts. 1987 Mercedes 300d Repair Manual
Great for beginners who want step-by-step photos. However, the W124 Haynes manual (often a UK import) is known for being a bit generalized because it covers too many different engine models at once. If you need a manual immediately to fix