As a 400E owner I completely sympathize with Duckmuck's feelings (although in total honesty I did not read all of your posts word-for-word).
When mine was just about to go out of extended warranty at 70,000 miles, a bunch of stuff went out -- starter, blower motor, fuel pump. I really started to wonder if I could possibly afford to own this beast out of warranty.
I now have 125,000 miles (not KM) on it and I have to say I am absolutely delighted I kept it! It has been very reliable since then.
I agree with Hidalgo that your key to retaining your money and sanity is to find a good independent mechanic. The reality of the situation is that SOME MB dealerships do NOT do a good job with customer satisfaction, in my (and MANY others') experience. The techs are wonderful, by and large, but the service managers and general managers sometimes are not.
Labenz is also right on target. Get a Topsider and change your own oil. I do, and it's one of the very few DIY things I do do. My dealership charges at least $85 US (and that was 3 years ago) and I would NEVER go back to a fast-lube kind of place.
Do an "Italian tune up" on it every month and it'll dramatically help your rough idle condition (unless you have a bad wiring harness). Find a SAFE stretch of road and just floor it until it hits redline on the tach. The first few times you do this it'll probably buck and snort like a bronco about 5500 RPM but if you don't lose your nerve it'll clear out that carbon.
The hard part is finding a long, straight, SAFE place to do this since when a 119 engine is near redline your speed will be pretty fast ...