MB hasn't published a CD manual for the R129 series so that option is out. You can get the W124 series CD manual that covers the M104 in the 1990 300CE 24 valve. Should be close enough. Plus you get the maintenance section that has procedures for the R129 series as well.
You can pay ~$25 for a year's subscription to alldata.com that has decent coverage of most cars. An alldata.com subscription allows you access to information on one specific car such as a 1990 300SL for you or a 1995 S420 for me. You won't have access to information on other year models of the 300SL or other 1990 MB models. Not a bad deal but you have to have internet access whenever you want to use it (you can print procedures) and the interface takes some getting used to (why are brakes not in the general menu but in the maintenance menu?).
There might be coverage of that motor in the Haynes UK W124 volume or either of the W124 manuals available at amazon.com.