I live in Potomac and have a 1982 300SD. Some of your problems are reminiscent of mine.
1. Vacuum leak causing runon - I replaced the vacuum pump (fairly easy DIY job) and the problem went away. I did check the lines, etc., and didn't see anything else. Since the car has over 200,000 on it, I'm erring on the side of replacing parts instead of spending huge amounts of investigative time.
2. "Plastic thingie" on door - I had that problem with my right rear and did the job myself. It WAS a bit of a job, but those are the ones that save you the most money as a DIY.
I would buy the 126 CD from MercedesShop and review the basics there.
On a mechanic, the best I can suggest is Potomac, Cherner Classic Cars, who is now doing Mercedes work. I had them work on my Packard (another story...) and they did good work at a reasonable price. I've seen a number of later model Mercedes there, as well. They are right on the corner of Falls Rd. and River Road. Lots of nice looking classics parked out front. I don't have any other mechanical experience in Potomac.