Do I need to buy a gamepad, steering wheel, or flightstick?
The Rift comes with a wireless gamepad inside the box, as most VR games use the gamepad for input. You do not need to buy a gamepad.
As for steering wheel and flight sticks, some games require them and some games play much better with them. You can review the list of such games yourself, but essentially, if you want to get into VR sim racing, then you will need a steering wheel, and if you want to get into VR flight simulation, then you will need a flight stick.