Although your company's name has been approved by ROC during establishment Mercedes Benz can send a notice if they have any objection to your using the name Benz for your company.
You will need to reply to the notice through your lawyer as your issue falls under trademark infringement.
The standard for trademark infringement is “likelihood of consumer confusion,” or for short, “confusing similarity.” This is a very different standard from substantial similarity under copyright law.
Trademark law is not concerned about whether artistic expression has been copied. Rather, trademark law protects how a logo functions as a brand name, that is, to designate the source of goods or services. A new logo is confusingly similar to the original logo if consumers who encounter it might believe that it represents the same company. This is generally analyzed by looking at the similarity of the logos, the similarity of the parties’ respective goods and services, and several other factors, such as the strength of the original logo’s reputation, the relative sophistication of each party’s customers, etc.
In your situation, your “service is completely different (and in a different legal class) than that of the trademarked logo.” This sounds like you might have a strong argument that your new logo is not confusingly similar to the original (depending upon how the other factors in the analysis would come out). Goods and services are confusingly similar if they are related to each other in the marketplace, or if they are the types of goods/services that are generally offered by the same company.
This should be the stand you need to take in replying to the notice from Mercedes Benz.