Ive installed HID's in both my cars.
Youll have to modify the reflector of the W124 headlights to cut out the glare.
There is a cutoff and it wont glare other drivers but the light produced on the ground isnt as great as the W140's lenses, but its still better then halogen.
From inspection of both real HID lenses and halogen lenses for the W140's the etched pattern on the glass are identical. So that leads me to believe they were originally inteded for HID use. The sharp cutoff remains, and the light is spread wide and far on the road.
Dont get that silverstar garbage, its actually worse then your standard halogen bulb because it has a tint film that gives it a blueish hue. Which leads to poor lighting.
Here are my threads.
HID W124
http://www.benzworld.org/forums/w124-e-ce-d-td-class/1296005-hids-my-w124.html?highlight=hid
HID W140
http://www.benzworld.org/forums/w140-s-class/1306971-6500k-hids-installed.html