I have had both a 320 cdi ML and a 350 cdi? Make sure it has a full service record , gearbox fluid in the 7 g box should have been changed at 75 K miles or 5 years, which ever came first . It should be a very smooth drive with imperceptible gear changes. Make sure it is cold when you take it for a drive , tell the garage you do not want them to have warmed it up before you arrive !
When gearbox is still cold for first mile ore two they can sometimes have a rather harsh change up to second or down 2nd to 1 st, a gearbox service should help .
Make sure that there are no dashboard warning lights showing, in particular mine both had ABS wheel sensor problems after being left parked up , this problem or low battery voltage will cause a series of dashboard warnings to be illuminated.
Engine should be quiet, no rattles even when cold and no noticeable transmission noises .
My daughter currently has a 2015 model and seems to get mid to high 20's round town / mixed driving and over 35,on long motorway runs.
Otherwise normal car buying rules apply , check it has good tyres, preferably all 4 same and with good tread , they are expensive as they will be 19 or more likely 20 inch rims so make sure wheels look good and no noticeable damage. try everything electrical out and everything works properly and check sat nav has been updated.Otherwise they are great cars and I enjoyed mine
