Don't know much about the 6.9 (other than they haul), but can give you a San Diego recommendation:
Rudi's ******** Stuttgart (619-560-5068). They're on Balboa Avenue, almost straight across from San Diego Mercedes.
They're an independent Mercedes-only shop. I had them do the pre-purchase on my car and they were very thorough. Have used them since for maintenance and am very happy.
One thing you can do with the VIN number: Run a CarFax report if there's any question about the origins of the car. www.carfax.com
Gives you extra peace of mind for $25...