If I were you, I'd try and get the Mercedes approved warranty. One thing to check though is the claim limit, I know the BMW claim limit is the price you paid for the car, so it's possible that any warranty might not cover the bills you're trying to avoid.
I was offered a WarrantyWise warranty by a dealer when looking at a car (they give you 6 months to cover themselves with an option to pay for more), but they said the single claim limit was £5k, which is ridiculous even for the cheaper cars they have. At the time, I was sitting in an office next to the Carrera GT they had priced up at £599k