Is this your case. 900D This is way over the top for air cooled components. This size/style of case is really for serious water cooling.
You certainly do not need fans on the underside of the case as you have more than enough air being blown in from the bottom side.
As for fans, Corsair cases come with decent fans but with the amount of fans you seem to have you might want to think about a decent fan controller as this is going to be one noisy build with this amount of fans.
Dependant on your make and model of GPU it is probable that you will not need any further cooling.
It would be useful to know what you have chosen for the rest of your components.