You don't need an airstone at all. They are just for looks and for extra airation, but this isn't needed for Betta that can breath air if the water lacks oxygen.
You should change water every week in a tank that small regardless of what filter you use.
You don't need the suction cup filter if you plan on using the UGF
. It is just an option in case you don't want to use the UGF
which most people don't like to use. And if you ever want plants UGF
's are useless.
If you use the UGF
that comes with the tank then there is no need for a powerhead. Bettas also don't like or need a whole lot of water movement as it becomes harder for them to swim with those big fins.