I'll give you two suggestions:
HBL Mercedes in Tysons Corner, has a German technician named Norbert. In addition to being an "old School mechanic, he is also a past president of the local MBCA section. Better still, he is a Pagoda owner!
Don Poole Service Center, on Lee Highway in Falls Church, has a good reputation. Don did some pretty amazing work reviving my mostly dormant fuel injection system when I first got my Pagoda.
Be prepared to spend good money for good work!