Looks like a nice ride.
I'd check to see if it has upgraded intake. While in there, check shaft play on turbo. Find service records. If MB 229.51 oil not used, expect DPF to not last much longer.
Can't be sure if proper oil change fill procedure used...look up TSB about engine noise and double check you don't have a shot front bearing from incorrect oil fill procedure
At its mileage, it will need tranny fluid/filter replacement, most likely shocks,brakes, xfer/diffs fluid replaced, etc...etc
Check airfilter, they clog easily.
Use anything you find as a reason to walk or low ball the dealer on selling price. Don't be afraid to walk away...use the same BS they would tell you if you traded in a diesel vehicle..i.e. diesel fuel is more expensive than gas and diesel vehicles not worth as much, diesels loud and unpopular, expensiv to maintain, then knock another 3-4k off the price