On a 95 E320 there is a 1 piston swing caliper OEM which I would like to replace with a 4 piston caliper from a 91 300ce. I am aware you have to change the rotors, but what else do you have to change to make it fit?

 

The 95 E320 comes standard with 4-piston front calipers, as do all W124 models with M104 or M119 engines. Only the M103 and diesels have the single-piston floatnig front calipers. You might want to verify what's on the car now, and/or the model/engine... maybe post a photo or two.