Looking under teh hood of a friends '84 300D turbo I noticed mounted to the pass side frame rail just under the air intake hoses what looks like a small electric water pump on the heater core feed line.
I am in need of such a water pump to circulate the hot water from my water block heater at night. Is that unit a water pump and if so what is thepart number and what models are they on?

It's the aux. water pump. It is used to make sure the heater core gets circulation at low rpm.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41NQ104MIO&year=1982&make=MB&model=300-DT-001&category=All&part=Auxiliary+Water+Pump

The later 300D/SD etc. have them.