My current A35 is my first foray into Mercedes brand and I've got to say I'm not impressed with their quality and reliability. Many people have said that they're, once stellar quality and reliability, diminished some time ago. I can't fault their design and use of technology though. I suppose many of us make the compromise to enjoy this “luxury” brand.If you buy Non-Standard Wheels for your Merc you will probably invalidate the 3 year warranty that you hold. It is always advisable to fit standard parts, even if they are expensive, that's the drawback in buying a premium car.
What is surprising is the shape of the Crack. I cannot see any other damage in the area to imply that an impact took place. Normally such damage on both Wheels would imply a straight ahead Kerb impact and not at high speed [probably less that 20mph]. The edge of the Wheel should show signs of deformation and the Tyre would have some damage because of the forces pushed through onto the wheel.