Originally Posted by Happy2rwe
So diesels before 2006 might be ok? Thanks for your reply Avanti
Speaking roughly, yes (IMO). But don't go by model year. Ask whether the engine requires DEF fluid. If the answer is "yes", I personally would consider it a significant negative, although not a disqualifying one (I am trying not to exaggerate the risk).
As for older diesels, most of them that you find are going to be Sprinters. You should understand that there are two quite different previous generations of Sprinter vans. The older ones are known as T1N and they came with a 5 cylinder engine. The later are called NCV3, and were introduced in the US in MY2007. They came with a V6 Diesel engine (there was also an I4 available in MY2014 only).
The T1N is highly-regarded as being relatively simple and bulletproof. It is however much more "truck like" in ride and noise than the NCV3. None of them require DEF fluid.
There as a short period in which the NCV3 V6 was produced without the DEF system (which Mercedes calls "BlueTec"). Many people think that the early, non-BlueTec NCV3s were the high-point of Sprinter design. If you manage to find one of them, it is worth seriously considering.
All a bit confusing--hope this helps a bit.