Janos:
Here are some pointers based on my experience with my 91 560SEL:
1. Driver's side door - your door switch is probably stuck to open position (it thinks the door is still closed?)
2. AC - I have no input on this..
3. Rough Idle - I would make sure that the electrical stuff is in good shape ( ie distributor cap and rotor, plugs, plug wires, etc). Then I would look at the fuel delivery ( EHA test, duty cycle test, idle control valve, temp sensor, throotle position switch...a lot of posts on these procedures). I once had a rough idle problem for a long time... I adjusted the idle at the tower as it was running lean, and replaced the idle control valve. I also found one plug to be improperly installed...
4. Vibration at about 70MPH - similar problem I had ..resolved problem by replacing motor mounts...the motor mounts on my car was badly wornout..
5. Guide rails - this is an expensive repair if they break and cause other things to break inside the engine. I replaced the upper guides and tensioner on my car about a year ago. The guides were discolored and slightly worn-out ( slight chain tracks were visible). The tensioner was badly worn-out. The metal backing was already almost exposed..

hoep tis helps..