Before filling the engine with new coolant be sure to replace the block drain plug and tighten it down.
Use only Genuine Mercedes coolant or an appropriate formulation if using another brand. These coolants are designed to be compatible with the materials used in the W123 cooling system.
Pre-mix the coolant and water in a clean container. Mix up a total of 3 gallons. On this car this meant using 1 gallon of coolant and 2 gallons of water, but it will depend on your climate. The system will take about 2.5 gallons of coolant/water mix at first, and then the remainder after some bleeding and driving for a few days.