I find a separate low voltage alarm (even though the kk2 kind of has one built in) is usefull.
Props - get 10 x 4.5 or similar for a 450 frame with 3S lipo
Get 8 inch props for a 330 frame.
Get spare props
Get a couple of lipos.
You will also need:
RX to kk2 servo connector wires
Battery connectors to match the lipo you get
Lights are useful for orientation
Lipo pack for the Turnigy 9x