I have a 1997 E320. I feel vibrations in the steering wheel at 50-60mph (not always). I have balanced (not rotated the wheels).
What could it be, wheels or bearing ?

Go Simple first. It could be a tread separation starting in one of the tires. Put the back tires on the front - get them balanced and see if it is gone. If it is have your previously front tires checked. When you take the front tires off - look carefully at the tread for any signs of distortion or out -of-roundness.

Does application of the brakes change the vibration? Does turning the wheel slightly effect the vibration?

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No, breaking does not change vibration. it is als not continuously, I remarked that it is at 55-60mph, downhill or when tha highway is slightly bowing. I also measured now tire pressure and one of the rear tires was 34.5 (???) and the other 32 PSI. Tomorrow I am also going to rotate rear to front. I have Pirelli 225/55 R16 95W, tires