I bet they'll charge maybe $200-250 for those if you take it to a shop.... I say do it, because that's what I had done, but I also had everything else done. looks like you want to do it yourself, so yeah it'll be alot cheaper... might as well!
the water pump and the tensioner are on the serpentine belt and yes, you should change them at the same time you do ur timing belt. its not the tensioner pulley that wears out but the spring on the tensioner!! You can see if you need to replace it or not by checking the tension on it!! if you can move it really easily, then yes, if you have to use like 50 ft.lbs. then no you don't!! just check it!! if you do need to , it is really really easy to replace!!