How many Steam games or other digital games do you have? I have a 1TB almost full of installed Steam games.
Another 1TB is 2/3 full of media: video, music, pictures.
I suggested multiple hard drives (in additional to SSD) because you plan to do video editing. Your rig is beastly, and using only one HDD will bottleneck it on video editing.
In terms of 'messing with it', a graphics card plugs into only one slot. Then you need to connect two power cables, and another small cable. It's very easy, there's plenty of YouTube vids for it, and we can give you directions too.
In any case, the rig you built above is pretty hardcore and will take all the games and video editing (streaming too) that you can throw at it.