I own three vehicles all with Bosch fuel injection. Based on my past experiences and to a much greater extent, Benzmac's, I would put the cold start injector at the top of my list. However, don't go down this path without a service manual. If the injector is simply leaking, then it's a simple diagnostic. But if it's not, there is an electrical logic path that needs to be worked through.
For this job my Haynes/Chilton's has enough info. If you don't yet own the Benz service manuals, get them (they sell them at this site on CD). They will pay for themselves within a job or two, not to mention the number of parts it will save you by not doing the "blind squirrel" method. Also, your public library will have automotive maintenance manuals. THey usually have the jobber type (the brand name escapes me right now).
OBTW, double check that the airflow sensor plate moves without friction. If the plate has a lot of gunk around it, clean it off with a good carb cleaner. The clearance between the plate and the body is minimal and sometimes gunk builds up and causes it to stick.
Keep us posted.
S, J.R. Brown
2000 G500 LWB Obsedian Black
2005 Toyota Tacoma Access Cab Off Road Sport
1993 Volvo 240 Sedan Anthracite
1980 450SEL Champange (owned it for 15 years. Great car)
1986 280GE LWB Anthracite (Sold it and kinda wish I hadn't)