Isn't Mercedesshop opening a tech/repair shop in Georgia? I would head over there if and when they do. Otherwise find another mechanic. I had two different mechanics tell me to junk my 1986 420SEL because they couldn't find the cause of a rough idle. They said (without doing any tests or even taking the valve covers off) that I needed a valve job that would cost 5 grand. Then they said they had seen valve jobs on these cars go as high as 13 grand because things inevitably break. I found myself a good mechanic who diagnosed the problem, replaced my fuel distributor and injectors and the car runs as smooth as glass now. There is likely a better answer for you out there.
1995 E420 - retired
1986 420SEL - retired