Hmmm.
Some of the stuff they may be looking for at any given time in the interrogation, sorry interview, may be.
Safety First. Your safety is paramount along with the customers and any pax on board at any situation.
Being utterly charming at all times. You will be at the coalface of "customer services"
Say as part of your research for the interview you have read up on forums about common faults in newer Mercedes. (After all you are not a member of any forum which may bash Mercedes from time to time.

You are very pleased there are so few faults on the newer cars. If they ask what fault bugs customers most say trim rattles. That throws it back to production not dealer inactivity or local main dealer tech's faults.
Scenario could be a verbal or as you say - having a non starter out the back for you to get going.
Newish vehicle requiring a jump start and making sure you dont blow any sams or fry ECUs.
Verbals - Its dark and raining and very late. You have a lady driver on board with two wee ones in child seats. Car will not start what are your priorities.
Keys locked in the car and customer needs to be away pronto.
Where do you see yourself in five years time.
Tell us your fix at the roadside success rate. (Its brilliant of course) If they dont ask tell them anyway)
Tell us the worst fault you could not fix at the roadside. (They should all have been "offs" - you have obviously got each and every "failed to proceed" sorted.
Tell us about yourself always comes up as does "Why do you want this job" Let them hear and see the passion of your dream in your body language.
Last time we met you were a smartly turned out fella and I'm sure you will wear a tie and your shoes will be squeaky clean. What ever car you turn up in you be noticed by the "panel" I'm sure.
The worst car you had was never a Mercedes but the worst car you know about was a friends BMW.
Maybe give Ian or Olly a ring to see what faults they are sorting on newer Mercs.
Good Luck Tiff and I hope not to see you in your professional capacity any time soon. I would have to buy a new car anyway.