I cannot speak for a W115, but i did the conversion on my 79 W123. There are two ways to do it, 1) convert you series setup (79 W123) to a Series/Parallel set up, this is done by removing the old glow plugs/wires and Grounding strap and installing the new plugs with new fabricated jumpers wires to each plug( Leave the Grounding strap off the last plug off,Cyl #1). This method will improve your starting immeasurably. I drove around for months like this and it worked fine, The problem with this is the plugs go off when the engine starts.
With the new relay kit the plugs stay on for 3 minutes after engine starts and this smooths out the idle untill the car heats up, works great. Because you car has five cyls you have to order the extra plug and you will need a extra relay to work # 5 cyl i think the part # is 0 332 002 156.
Yes, the kit comes with instructions.
I hope this helps but remember mine is a 79 W123.

__________________
1979 300D.