The FI system on these cars is basically CIS with some electronics wrapped around it. The ECU is fused at the OVP, OVervoltage Protection relay which is mounted between the two firewalls and has a red top and a clear hinged top and a fuse or two. The ABS computer is also mounted between the firewalls and is not to be confused with the ECU behind the passenger kickpanel.
This page will give you a start on the CIS-E system
http://www.landiss.com/mixture.htm
There are many posts on this system. Be aware that the CIS fuel distributor and the feedback pot are getting long in the tooth and are prone to failure/wear.
560sec.com is now under construction and the buyers guide is N/A.
There are lots of good posts on what to check including the DIY section. Check benzworld.com and the MBCA forums as well.
http://en.wikipedia.org/wiki/Mercedes-Benz_W126
The CE light is really just for an alert to change the O2 sensor. I really suggest you get the service CD's usually available on Ebay.
http://www.classictechlit.mbusa.com/MB/cstmsearch.asp?MYID=1040&SDID=1332&MDID=1237&ENID=1069&catalog_name=Retail
the technical data book is also good.