Dont take this the hard way sir, but its possible you were driving an abused vehicle. The engine and car may not speak the same about the condition as a whole. Heavy on fuel? I mean you bought a merc, used and not very new so you may never know what the last guy did to the engine.
I've driven quite a few merc's and its quite obvious when you sit behind the wheel of a well kept car and one that was just treated like any other donkey cart. I cant comment on all the electrical gizmo's, never had such problems with the merc's in our family. But I will disagree on the comfort issue. JDM ALTO has the worst seats you can get in a car. they are small, flat and hard offering no support.
The early 2000 mercs did have quality issues and yes I've read them time and again on MBWorld but not to the extent that one starts comparing a benz to a alto.
Anyways, I own a 84 w201, bought new and still rolling. Its clocked 140k's and has quite a history with Shahnawaz (though I do alot of the things myself since its fun and you can be extra careful). The car runs like a champ, drives like a benz and till date has NEVER broken down on my dad or me.
Its all about how you treat your car, how you maintain it and just enjoy it.
Cheers. Hope you buy another merc soon and really feel what a merc is to drive like