My water pump started seeping coolant and I will replace it. I did notice that the belt tension was a little on the tight side and the pulley runs a little off center. I loosened the belt a little and it stopped leaking when taken on a day trip. I read the procedure in the FSM and it looks straightforward except for one question.
There is an aluminum base between the WP and the block and it has a horshoe banjo fitting on it going to the head. Is this part of the WP, and should I replace the gasket on it? What fluid does the banjo line hold? Replacement of teh banjo washers, of course, if I remove it.

Thanks

The "base" is the water pump housing. No need to remove it or the vent line. Just remove and replace the pump itself.

Engine coolant.

Is regular green coolant ok? I remember something said in a forum about only using MB coolant.

Idealy you should be running MB or Zerex G-05. But green will suffice since you have been running it and to properly put the good coolant in you would need to do a complete citric acid flush.

The Mercedes Benz / Zerex coolant is formulated to protect the plastic and aluminum parts in the cooling circuit (I believe so) and the Green stuff only protects the aluminum and degrades the plastic.

Citric acid is used for removing significant rust/scale buildup. A good H2O flush is adequate to accommodate a change in coolant type.

I think I will just use water as I don't want to wash away the old crud that could be sealing the head gasket.