The usual recommendation is to change the water pump "while you are in there". Can't say when the last time I heard anyone complain that their water pump went bad though.
When I had my Subaru timing belt done a couple of years ago the independent shop was almost as expensive as the dealer. But the indy guy was going to replace the tensioner along with the belt. So my guess is something like that is what your usual mechanic is referring to - idler or tension pulley. May not be entirely necessary but could be "cheap" insurance.
And yes, both the indy and the dealer included the price of the water pump in my timing belt maintenance so I did it on my Subaru.
Best thing is just to ask your mechanic exactly what they recommend and ask the other shop if that stuff is included and if not, why not.