There are some dissimilar metals in the 123's. like the water pump is mounted to the aluminum housing, corrosion can occur there, and there are steel pipes that carry coolant into the heater.
I believe the MB solution has been tested and has the right additives to reduce corrosion in these cars.
Granted there are certainly some equivalent solutions available, I heard Peak antifreeze is similar to MB mix, but for $10 or so I'll buy the MB stuff and call it cheap insurance.
Changing fluids regularly is necessary regardless of what brand you use.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!