Here is my current rig:
This is clearly a high-end build; I am a software engineer, and I work on this machine all day every day. Here is what I would recommend for a decent video-editing system, at a medium cost:
If this lower-priced system appeals to you, but you want to upgrade one part, I would recommend a more expensive video card.
But the most important part in these builds is the SSD. I will not build a system without one, for anyone, for any reason. They make such a huge difference that I can't bear to use a computer without one.
And while they are much less expensive than they used to be, so are spinning disks. So don't store your video files on the SSD; 4TB drives are $125 to $150 these days.