You're right about the C-Class being built to a price. The quality of the plastics is much inferior to those found in the bigger cars. One of the problems with the door trim is that a cheap rigid plastic is used for the side pockets; this creaks and rattles as it expands faster or slower than surrounding materials while the car is warming up. Drives me nuts!
As for spec, my car is not too bad. I looked at a C180 Classic and that really was poverty spec! As you say, power locks and that was about it. My car has the Sport trim package, manual air conditioning, 8 speakers, electric front windows (but I never sit in the back so that's ok) and a few other bits and bobs.
The most annoying rattle comes from the B-pillar on the driver's side--right next to my ear. I think it is something in the adjustable seat belt mechanism.