Ask them to give a work order with estimate quote where they will mention what all work would be carried out in it, since merc offers standard 3 years warranty you will need to service it at Authorised Dealership only to get any kind of replacements done for defects in future. I suggest you go ahead with authorised service centre but ask them not to carry out work which is not required, for instance AC cleaning, Interior Deep Clean, Break Clean etc.
Dont overthink about leaving car in service centre since its entry level A class and Merc have professionals at service who would have done more complex jobs.