I have a `98 C230 and live in the mid atlantic where they salt roads, and find a garden hose does a great job of cleaning the underside. In the winter I reluctantly use a self service car wash wand - and I worry that the spray is too powerful.

in my factory service manual it does mention something about having the underbody inspected at a recommended interval and any damage to underbody rust coating repaired.
Also it mentions when having the engine cleaned afterward always have it sprayed with Mercedes Benz anti-corrosion wax, all linkage protected with lubricant and the poly-V-belt and pulleys protected.