i just have mine replaced and I have a W140 with a 3.2 engine. M104.990 engine petrol car.
The regulator is cheap. I got a mechanic to replace it but I watch him did it in 5 minutes. If we have the same engine, we should, I think you can do it yourself. The part when I looked at fastlane was 40 odd dollars...
Arthur is right, you should get a pressure meter and check for leak. Without any tools, you can pull the little hose off and see if petrol drop out. if you have, its a replacement.
My tech had to use a special plies to remove the ring. Without that it would be difficult but can still be done.