I don't know much about the W124 but if you go to
EverythingBenz - Mercedes-Benz Forum and Web Search Using Google you can see a stripped down version of the EPC. There you can find pictures of the parts that will show if there was ever a non SLS version out there. I'm guessing there was... as there was for the W123. Anyway carefully check each part number between a car with SLS and a car without as you might find the springs are different for example. You might have problems getting all of the part numbers using this Russian EPC but I'm sure with help from the forum and google you'll get the whole list in the end if you don't want to go through the official route via
www.startekinfo.com (Non US residents have to pay for this service - I'm guessing 'cos you used the word "estate" instead of wagon that you're in the UK)