Well either way its impressive as insuring a mercedes at 17 years old is not exactly cheap if the car is cheap or not is only half of it. running costs / insurance etc is the hard part hence why most 17 year olds get 1.2's as they are cheap to run and normally all they can afford such as myself.
Again...that's just a matter of having the money. I would be impressed if you have worked hard to afford it (which you probably have), but I would not be at all impressed by a 17 (or 19) year old driving a Merc if dad had paid for it. I'm sure you will agree.
I know of a 20 year old near me in a new Aston Martin...an impressive car...but he didn't pay for it.