On your first water pump replace, don't just get the impeller, go ahead and get the kit. It is likely you will need one of the other items, go ahead and have it to start with. If you don't need it then, keep 'em and NEXT TIME just get the impeller.
At my first change, I always do the kit, that gives me a reasonable base line. If the old stuff looks good, I keep 'em.
As mentioned, the impeller looses a lot of "Flex And Flick" being bunched up in that housing over time. I like to note direction and lube it well for that initial start up.
Oh, and then there's the thermostat...