Hi Philip
I've got an 05 ML 270CDI Automatic. (5 seats not 7)
So far (only been caravanning this year) its been a pretty good car to tow. Its got a massive towing capacity and will take most vans in its stride.
On the motorway you can click on the cruise at 60 and let the car get on with. It will take any hill in its stride and not waiver at all. You hardly know the vans on the back.
As with most Mercs the seats are very comfortable , I'm 6ft 2 and have a bad back , its the first car I've had that I dont feel any pain when driving and can get a near perfect driving possition.
Running costs , as with any 4x4 will be high , I generally get about 26-28mpg commuting to work (and thats in stop start traffic). Towing I'm getting about 22-24mpg , towing a van thats 1400kg+ loaded. Added to that we have 2 large dogs that weigh in at 60kg in the boot.
As for reliability , like most cars you will hear good and bad. Early MLs were seen as unreliable and this was somewhat claimed to be that they were built in America. However the later models seems to have a far better reputation , I guess having had the niggles ironed out.
As mentioned I've got an 05 , which is the run off model prior to the new version , so alot of the cars were coming with options as standard , i think they badged it as LUX(i.e leather , electic memory seats etc etc).
In all I'm very pleased with the car and the MB dealer I purchased it from.
Do you actually need the 6/7th seats? as mine doesn't have them I cant give advise , but the dealer steered me away from a verion that had them. Apparently there's next to no room with them fitted and its something I seen commented about in other forums.