I don't know too much about PC's, at least not as much as some of the other users on this site. But from my experience, WoW doesn't need a fancy, high-end PC to run it smoothly.
This is my current setup for my PC:
* Corsair Obsidian 750D Case with Window
* Western Digital WD Black WD2003FZEX 2TB
* Samsung 850 EVO Series 250GB 2.5in SSD
* Corsair Vengeance LPX CMK16GX4M4A2133C13 16GB (4x4GB)DDR4
* Razer BlackWidow Chroma RGB Mechanical Gaming Keyboard
* Corsair Hydro Series H80i GT 120mm Liquid CPU Cooler
* Corsair HX750i 750W 80 Plus Platinum Power Supply
* ASUS Z170-K Motherboard
* EVGA GeForce GTX 980 Ti Superclocked+ ACX 2.0+ 6GB
* Intel Core i5 6600K
With my setup, I can run WoW on ultra without experiencing any dips in FPS, the same goes for most other games as well. Most recently I bought Battlefield 1 and am playing it on ultra, experiencing no FPS drops. With saying that, you can buy cheaper components and still get the same results as WoW is not a very graphics heavy game! I hope that this helps out! :)