LIKE harpsichords and hard drives, the latest Mercedes-Benz E-class is about enduring traditions co-existing with an evolving high-tech world. Recent models, while progressive in many ways, have lacked the unimpeachable quality of their 1980s predecessors, so the Germans have set out to address that by looking back at what made the old W123 and W124 Benzes such enduring icons. The result, as tested in the E250 CDI, is one of surprising ability and genuine quality. Mercedes, at last, has finally sorted itself out.