HI there,
some manufactures do a serpentine kit or you can just get the tensioner as a separate item, the water pump is a good idea some do it some don't, if it was mine i would. As for the labour charge, this is a vexing question, as small businesses do relay on the margin on parts to help make ends meet, you may find that they will load the bill a touch to compensate, but i hope that they would do the right thing and not.
Mark (mhpautos)