I would have to agree with your mechanic. I and others on this forum have replaced things like the fuel check valve, accumulator, filter, plugs, rotor and cap partially in pursuit of solving this problem and I am narrowing it down to the leak down.
You might want to check your Overload voltage relay as well before doing anything.
Also do a search on this forum for 'warm hard start' or 'hard start' some good advice from Benzmac and other techs here.
88 230E (W124)