A noise that comes from the bell housing at idle could indicate a worn throw out bearing. A hard peddle could also be a pressure plate going out. You can check the wear on the clutch by checking the shaft position of the slave cylinder. The gasket has an area to slide in a gauge that is a go no go approach. The shaft of the slave cylinder has a reduced bore section. If the gauge does not go in, the clutch is worn. Of course i have no idea on the dimensions of the gauge or the MBZ part number. I don't believe any stiffness could come from the fork lack of lube. My guess would be it is time for a complete clutch job. Pressure plates are weird. There is no real way to tell that they are worn unless a finger is broken. I remeber having the tranny out of my w115 220d in 45min from when i drove my car into the garage. Granted that was over 35 years ago and i was young and energetic. Good luck