It's been my experience that you will not need to flush the cooling system if you avoid using tap water. The worst thing that you can put in your cooling system is tap water which could contain all kinds of junk.
My last Mercedes was 18 years old when I sold it and there was never a problem with the cooling system. I simply drained all the coolant I could get out of the system from the drain on the radiator and refilled with a 50/50 mix of distilled water and MB coolant. I've continued this approach with my 13 year old 300E and it runs quite cool in the heat of Arizona summers.
In very hot climates you could use a bit more water (55/45) but if you go too far you will then wind up with water pump problems because you will not get enough lubricant and other additives from the coolant mix.
I usually drain yearly because it's so cheap and easy to do.