I just installed these in my W115 and it really is the best upgrade. The car starts in 3 to 7 seconds and no longer hesitates when cold. I got mine from Kent Bergsma:
http://www.bergsmamotorwerks.com/
He is a good guy and also sells on E-bay.
Randy is top notch, too, and Parts Shop may have them.
Let me know if you would like pictures from my installation. The instructions that come with the kit have little relation to reality. For example, it is much easier to wire directly to the starter switch, than to pull apart your fuse box.
Note this kit is really for upgrading early W123 and W115 to the late W123 style. I did not see what chassis you have, but there is no point in the upgrade if you already have pencil glow plugs.
Cheers, Andrew Seidel.