The mounting in the dash for most recent Mercedes radios is DIN so a standard single DIN sized radio will just slide into the slot. So from this aspect, a lot of radios will fit. "Keys" can be inserted into two little slots at the bottom of the radio to remove it. You can buy the keys for a few dollars or you can make your own.
The bad news is that some models in you time frame had two piece radios with a head unit in the dash and an another box in the back of the car.
The E class in this time frame had this but I am not sure about the S class. Even if the radio fits, the wiring would not allow for a direct swap. The good news is that Becker has cable sets made to use other radios so you might check with them.
If you have a cd changer, most later models will not work with an earlier CD changer. In particular, most of the 99 and later models have a fiber optic interface that is completely different that the earlier models. This is also true for phone systems integrated into the radio system.
Perhaps you should try listing this on the audio forum. You can also search that forum as well and a lot is probably already answered there.