Class E Diesel - Help please

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One of the members of my family is looking to buy a used mercedes diesel and has asked for my help as I know a bit about cars although I am not familiar with mercedes(I know BMW and Porsche relatively well). The car needs to be diesel, estate with low mileage. I have looked at the mercedes website and I am a bit confused with the different engines. The choice for diesel engines is I understand, 220, 280 and 320. I read in this forum that the 220 diesel engine may be a bit too slow especially on motorway. My questions are the following:

1. In terms of performance and driving is there a significant difference between the 220d, 280d and 320d engines ?
2. Has there been any significant modification (engine or otherwise) in the E Class model since 2006 (one of the car I am currently looking is a 320d model first registered in 2006).

Thank you for your help.

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OK thanks for all these posts. Is my understanding correct that the diesel engines will change in 2010 ?

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Thanks - is the W212 engine already on the E Class Estate series or is it only on the saloon version ? Also what is the difference between the W212 engine and the current diesel engine for E280 and E320 ?