Not sure where you're living and you didn't mention wether or not 'the folks' are an official Mercedes Benz outfit. But, generally speaking, I think you're taken for a ride.
Go to another workshop and ask for a 2nd opion with respect to your front discbrakes and brakepads. Tyres should be available for much lower prices anyway, assuming you're running standard sized wheels.
Isn't there an internet community of C320 enthousiasts in your country
If there is they will be more than happy to share their experiences with you and give you practical advice with respect to you problems.