You don't need any storage space.
It's just up to you how much you want to have. Only things taking up space will be games, DLC, and stuff like that.
Games are like 7GB max (+ possible updates, DLC,..), and two disc games can a bit more than that, not quite double the size though.
There are no "RGH essential files", or whatever, that would take up a lot of space.
There's way to load game backups from network now, so I would not recommend put too much storage on the console itself, put the storage into your NAS so that can be shared with your other console's like PS2 PS3 Wii Wii U PS4 etc