Someone hit my car from behind I took to dealer that I bought from question is I have the power to choose repair shop I want?

It sort of depends on the insurance company.  The majority of companies will allow you to have the car repaired where you want, however, you must get prior approval first.  With the prior approval, you will be able to have any supplemental damages covered as well as have a warranty for the work.  In most situations, if you just go out on your own without the permission of the insurance company, there is no warranty on the work.  I would have to presume that with a mercedes, the insurance company will allow you to go to the dealer, as they may be more of a specialized vehicle.  But check with them and make sure, because the dealer is usually more expensive