4.2 liter engine questions

I have a F-150, 2003, with 104,000 miles on it. I change the oil every 3000 miles, no problems there. Engine is sound. I need to change the serpentine belt, and I need to know if the water pump should be changed also. I figure it does because of the new belt tension on the old shaft might cause failure at some point. Tell me if I am headed in the right direction here. Nothing is leaking at this point, just some preventative maint. Going on! Also, Anyone have any experience with K&N air intake systems on this engine? I am considering putting one of for 200 bucks.