Per the BBB, there are 3 different thicknesses for the rear (8, 14, & 20 mm). Unless your car was originally setup for a harder suspension, bad road conditions, the BBB narrows your choices down to 2 of the 3 thicknesses (14 & 20).
When I replaced my rear spring rubbers last fall (upper and lower), my spring colors were not visable either. Based on the above, I selected the thicker rubber size. At the time, I was only ordering parts through Buds Benz, and they only had 30mm pads (not sure where the 30 came from???). I installed these and have been very happy. Car seems to sit perfect (for me...), not too high, but mainly not sagging in the rear.
If you're going to do the work yourself, make sure to have a strong floor jack, and one that goes pretty high. I recall having to really jack up the pass. side in order to clear the muffler, tail pipe area. Drivers side was easy.
Note: I did not have my car re-aligned after I installed the new pads because I dont know who I can take it to and trust that it's done right.... Dont really notice any alignment issues...
Good Luck!
bpossel
Memphis, TN.
1971 280SL
1997 E320