Primarily I want a water/oil separator. They have a cleanable stainless steel mesh screen in them. Something I can visually check and drain ocasionally.
I have the heavy filtration in place in the original fuel filter itself.
The lack of one on these water separators on old mercedes from the factory was a real error in my opinion. From time to time I always got some water out of my diesel volkswagons separators.
They even have warning system that water is present in them. In theory you could get one from a volkswagon diesel in a wrecking yard. They are located just around the drivers back wheel bolted to the floor underneath. Probably as cheap as your going to get.
May have a hard time finding a good location on our old 123s to install it. Generally if you hooked up the water present warning system it could be located out of site like on the vws.