Agree on the fast SSD advice, but for video editing I would definitely add that graphic card is one of the most critical components. Hardware acceleration on a powerful GPU makes a huge difference to the speed of working with video editing (applying effects, quick high quality preview, fast rendering times). I have an ok powerful Geforce 970 4GB card, not the latest and greatest but makes a very significant difference on speeding up video editing a lot.
I cant find that article anymore but somebody actually did an evaluation and found that a gaming GPU is actually an overkill and doesnt result in any performance boost for rendering/transcoding videos. I think as long as the GFX card supports 4K resolution and has decent VRAM you are good to go.
P