The Rena is a canister filter, correct? That means there should be space in it for really any kind of media you want. Ideally, you'd want something like filter floss and ceramic rings. The rings are a GREAT media for growing bacteria and the floss filters particles out. As the floss becomes...yuckified...it can be replaced without really negatively affecting your bacteria colonies. Thats one reason why canisters are great. However, you can get by with just rinsing the floss out for quite a while. I've only changed mine completely a few times in 3 years.
As for the whisper, which i think is an HOB- you don't really ever have to replace it. A good rinse in tank-temp water (use water from a water change) is fine.
You need test kits for ammonia, nitrite and nitrate. Nitrate is the one you'll use most frequently after establishing your tank because it is the measure of how much waste you have in the tank. Ammonia tests test for the ammonia that fish wastes create (the first stage of cycling) and thsi then turns into nitrite, which is very very toxic to fish.
You can add new fish after the tank has cycled - but only a few at a time because you don't want a lot of ammonia and nitrite in there.
I wouldn't use the undergravel filter, myself. I think they tend to be more trouble then they're worth (they trap a lot of gunk under there)...but too late now.
You cannot overfilter your water. However, you will only colonize enough bacteria to feed off of the limited amount of waste you have. Plus, you don't want to create a whirlpool.