A used Mercedes is an amazing value: a world-class luxury car for a lower price than a new Corolla. But buying the wrong Mercedes is a world of hurt. Steve Sorensen, a Mercedes service center owner, describes the surprising rules of buying a worry-free used Mercedes. Newer models aren't always better, powertrains aren't the most expensive thing to fix, and higher-end models aren't more troublesome. Buying a secondhand Mercedes can be an extraordinarily good deal, if done right.