The miles are fairly high for an 09 registered car but that won't matter a jot in the long run.
The guarantee is transferrable to the new owner and is three years on UK supplied and registered cars (two years in Europe).
The price is right for a car of that age . MB dealers would be about the same and you can see quite a few by going to the MB website and checking out 'approved used'.
However, there are very few 220cdi's with such a great spec. COMAND, leather etc etc. In the approved used search engine there is a section on 'features' where you can put in auto, COMAND and leather etc. Very few come up that meet that spec and all are dearer if they do. If it hasn't been pranged etc, looks a good price.
Servicing on that age of car is every 15,500 miles or once per year whichever comes first. Check it was done at or close to 15,500 miles. Going over by too many miles can infringe the warranty.