I have a '98 E300 and have replaced the fuel plastic fuel lines twice. The first time was simply because I had the intake manifold off to replace the glow plugs and did it because the lines and shut off valve were exposed. The second time I really had a leak; it was from a crack in the shut off valve.
1. The shut off valve is attached to the injection pump with torx headed screws or bolts. I needed a torx socket to remove it. I bought a Craftsman EVO socket set (7 sockets) for $12 or $15, can't remember which. The socket that fit was adequate for the job.
2. There are only 2 O-rings that you have to buy. One is where the s.o. valve fits into the injection pump; it is black and about the same size as the one for the pre-filter. I understand that that is the O-ring that most often fails. The second is a small lime colored O-ring that is on the banjo bolt that holds the plastic line that goes from the s.o. valve to the fuel filter. Be sure to buy one of those it you are going to replace that plastic line. All of the other O-rings are already installed in the plastic lines.
3. I think it would be easy to mix up the lines and where they go into the s.o. valve. So be careful.
4. My leak looked like it was coming from the electrical connection too, but it was from a crack in the valve. From what I have heard, the most likely source is the O-ring between the s.o. valve and the injection pump.