Sorry,i thought you meant the upper part of the throttle body. once that is off everything becomes clearer.
I understand what you mean aboutr community garages,i'd go nuts if i lived where i wasn't allowed to work on cars.
So far you havew nailed down the majority of things that can go break and cause running problems.The biggest single thing you can do with those cars to make them run better is to DRIVE them.
The design of the valves etc ensures that the engine runs better and better the more you use it.
if you want to go ahead and buy a idle control unit and it's electronic box,this guy has them cheap on ebay second hand. I've bought from him before and his prices and Knowledge are very good.
http://stores.ebay.com.au/REDLANDS-MERCEDES-PARTS