You need to be looking at 05 onwards due to the rust issues and it must have service history.Make sure you already have a merc van experienced mechanic in place for servicing/repairs with the diagnostic savvy as you DO NOT want to be going anywhere near a Mercedes main dealer with it.
Then google "vito black death" as it will take too much typing for me to explain it.Sounds bad and it is when its happened and injectors have a habit of breaking during the clean up process.A main dealer will just tell you a new engine is the cure. Yet a set of copper washers(about £2) as part of preventative maintenance will ensure it doesn`t happen.
Electric gremlins will have your local garage scratching their heads and packing you off to the main dealer.
An example,key in the lock but nothing happens,no power up of ignition.Down to merc,new key,new ignition ecu,van will be off the road a week whilst the new key is ordered,arrives,then sent away again for coding due to security issues,£1000 and a week later it starts doing it again.It can be as simple as the electronic steering lock at the base of the column not disengaging and this tells the ecu to not activate the ignition system.
Drivers electric window not operating(common),thats a £600 fix with merc labour and vat.All the sellers with this problem tell people its probably a loose wire!