After the dealership has had a chance at it, I would ask how much is actually leaking? Are your engine temps affected at all?
With my 300E, the expansion tank went first ($85 fix at dealer), followed by water pump, then radiator. It took six months for the entire cycle to occur.
Your water pump will "weep" as the seal deteriorates, and then ultimately fail with a telltale sign of substantial amounts of coolant on your garage floor (your belly pan won't hide THAT leak). Because of the water pump location, it is difficult to trace the weep hole leak.
If your radiator is original, expect it to fail eventually, as they crack at the joint where the inlet hose is connected. If ultimately, the water pump has to be replaced, get the radiator done as well.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car