I don't know your car so I can't help with the specifics, but on an 1987 420 SEL with around 14 quarts of coolant you need 3 boxes of the citric acid that MB sells for about $16 per box or .5 kg.
The dealer needed to order it so don't expect him to have it. I would try to find it at a drugstore or chemical supply store (citric acid or lemon acid).
You need to have the mixture circulate in the car for 15 minutes at about 2,500 rpms.
Then flush and fill with proper antifreeze mix. I needed a little more than 1 gallon for the minimum mix.
I used hot water to mix the stuff and filled it through the expansion tank.
I don't know where to drain your block.
You might want to get a new cap if yours is old and might not hold full pressure anymore. I learned that my still original cap had been "upgraded" by MB to hold a higher pressure, which helps with running it cooler.