Any high-end workstation will work but I'd look more to the gaming side of things as they usually have nice video cards. If you edit with Adobe Premiere then stick with a GeForce video card. A 900 series video card would be nice.
For a CPU I'd go for a late generation i5 or i7 if you can afford it. There are also some great AMD options. I personally run an AMD 8-core rig and love it. Apps like Photoshop, Premiere, After Effects, etc... can use multiple CPU cores.
16GB RAM is nice, 32GB is better.
A 3.50 GHz, i5, 16GB DDR4 RAM, GeForce GTX 960 Video Card, 1TB hard Drive, will set you back about $979 at newegg.com. This and a big external hard drive will let you handle 4K with ease. Of course you want a 4K monitor too. You can pick up a really nice Samsung 28" 4K monitor for about $500.
Nice to have stuff...
A Blu-Ray disc burner will let you burn your own Blu-Ray movies that look fantastic on a big screen. Use it for back-up as the discs hold 25GB each.
I have a couple of SSD hard drives. SSDs are solid state drives which means there's no spinning platter. They are quite a bit faster than traditional hard drives but cost a lot more. I use on SSD for my Windows operation system and another one for my applications. My system boots insanely fast and apps like Photoshop and Illustrator are crazy fast.
Everything is going USB 3.0 now. Get an external USB hard drive and SD card reader. There's no such thing as too big of a hard drive. Don't forget a backup hard drive just in case...
Buy a mechanical keyboard. Trust me on this, once you use a mechanical keyboard you'll never go back to spongy, crappy keyboards again.