Having had several E class estates I would suggest you might do better to find a late E270. The older engines are more frugal but still have massive torque.
The newer V6 diesels are quite a bit more thirsty for not a lot of torque gain, just more BHP.
Avoid the 4 cylinder engines of all age for such a heavy van. They're fine on the flat but getting up to speed or up hills is a noisy and unpleasant experience. (unless you have the patience of a saint)
Also, the earlier engines had a 5 speed auto compared to the new ones having the 7 speed. 7 gears sounds better but in practice the car is never in the right gear at the right time.
My E270 returned over 40mpg solo and about 30 towing. The E320 V6 CDI was about 25% less. The E500 V8 petrol was unspeakably bad when towing, (12ish to the gallon) although a damn fine machine when solo.
Hope this helps.