Not everyone here is an expert but at least they are trying to get the information and trying to help someone else. Some information may not be correct but usually a more experienced member will politely indicate this.

Regarding the diagnostics many "newbie" owners don't understand their car's systems and don't have the tools to perform the diagnostic procedures. While its not the "correct" way to service your car it often works for many people at a considerably cheaper price. Technically the correct way to service your Mercedes Benz is to turn it over to the dealership and let the factory trained technician service it and pay the bill when you get the keys back.