Every brand has their lemons. My lemon was a Jeep Grand Cherokee during my younger years. I haven't purchased a Jeep/Chrysler product since nor will I ever. Though I'm not a fan of MBs (my wife likes them), I've been fairly pleased with their reliability and their service departments tend to be top notch. I'm on my third Audi and they've all been trouble free thus far, knock on wood.
In regards to the OP's dilemma, I would not buy a car (used or new) if there are any sort of indications that it's been driven hard during the break-in period or abused/driven like it was stolen. Our first GL450 was the manager's car with 500 miles on it. It went in the shop 6 times within the first 4 months of ownership with fairly major problems-leaky gaskets, transmission issues, radiator leaks (replaced the radiator), multiple CELs, fuel system issues, front end squeaks, etc. More than likely that dipwod abused the crap out of it.
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