I don't know how much coolant is needed to 'flush' the system.

But to 'fill' the system, the owner's manual for 1989 W124 says the cooling system capacity is 'approx 10.0 L (10.6 US qt)', with these numbers for the mixing ratio with water

Approx freeze protection: -30C / -22F
MB antifreeze: 4.50 L (4.8 US qt)

Approx freeze protection: -45C / -49F
MB antifreeze: 5.50 L (5.8 US qt)