i'd definitely buy a 07 or after e320 bluetec or e350 bluetec se

mercedes diesel technology is 2nd to none.  it's superb workmanship, longivity, comfortability, fuel economy and forever stylishness will warrant your satisfaction for next 20 years.  the best deal may not come from buying a new car but from buying 1-2 years old with low mileage and well cared for.  40k enables you to buy a 2011 e320 bluetec or 2012 e350 bluetec, depends on where you are.  check craigslist.org and have somebody knowing car well help you. good luck!!