One problem I've seen with a lot of the mid-range M-Bs that are more than say 5-6 years old is the headlights get very cloudy. That just looks cheap, like a cheap 10 year old American car. I wish with all their quality they'd figure out how to engineer better headlight lenses.
Headlight lenses are an issue on all new cars, some more than others. MB has stepped up their quality with the new W204's when compared to the previous generation C-class (actually, across the board compared to their early to mid-2000 models). Every model of car can and will experience some failures, and hopefully you are not the unlucky one to get a lemon (regardless of what brand you choose). Regardless, any good detailing shop can easily wet sand and polish head light lenses in place to make them perfectly clear again for under 150 bucks as necessary!
My family has owned all kinds of cars: American, Japanese, and German. I have personally worked on all different kinds of makes and models for family and friends, and I can't say that these cars are any harder to work on than my last Japanese car (which didn't cause many problems, but in the end left me unimpressed to the point of getting rid of it).
The W204 C-class is generally a well-built car. My brother has a 2009 BMW 3-series, and his has caused more issues (although minor) than the 3 MB's in my family have all together. I find routine maintenance and replacement parts from BMW are far more expensive than the same parts from MB.
So far, oil filters, wipers, and other common items are not marked up terribly from the MB dealers in my area, and are quite reasonable I find. The servicing is more expensive, but I only do one service every 20,000 km (13,000 miles). This more than makes up for the expensive service, especially if you do them yourself as I do. The car will tell you what needs service and when (if you enter the special service mode using a sequence of buttons).
For the few very minor warranty issues I've had, the dealer in my area has been a pleasure to work with. Mind you, the MB dealers in the Toronto area are all owned by Mercedes Benz Canada, so they are corporate stores. That might make a difference, and your mileage may vary
In terms of the cars, if properly cared for and serviced regularly, they will mechanically last longer than you will care to keep them. Electronics can always break at any time, or they can last forever. This is true for any new car. Be careful which options you choose, and the simpler the car you buy, the less potential issues you will face down the road. I personally have not had any issues in that regard as of yet.
Good luck with the decision