I've read this thread with interest, having just bought a second hand C-Class myself (though only a C200 as against your C280, I'm a bit jealous).
Like you, I have a certain paranoia that my car has been in an accident. Every rattle (and there are 2) gets me thinking, every miniscule discrepancy in shut line width has me worried, every time I hear a slight tinniness when I shut the door... you get the picture.
The reality is, my car is 2 1/2 years old and has the rattles and defects of a 2 1/2 year old car. It sounds like much the same could be said of your car: if it has been in an accident, then that accident was very minor, perhaps a parking dent to the door or a clumsy traffic manoevure requiring a new door skin.
At worst, your door is a bit iffy. It's not like your chassis is out of whack or you have a biffed up B pillar. At the end of the day you could get the door completely replaced relatively inexpensively.
That said, I know how a minor niggle will ruin every drive.