It is super easy to customize and build your own PC these days, especially with the help of a nifty little website here:
www.pcpartpicker.com
The site allows to you to view the prices and online retailers of pretty much every computer part out there, and shows you any potential compatibility issues as well as the total power draw of all the parts. I built my PC this past summer using this website and found the process to be really streamlined thanks to the site allowing me to see what parts worked well together.
You could easily build a computer for about 600-700 dollars that can run Civ VI on max settings at 1080p/60 fps. A GTX 1060 or Radeon RX 480/470 paired with a core i5 would probably be your best bet for that type of performance.
Also, if this is your first time building a PC, don't be worried, it's not difficult at all really. The build consists of knowing where to plug in parts ala Lego style, and the only tool you really need is a magnetized screwdriver. Plus there are Youtubers out there who will post an hour long video showing the build process so you can follow along with them as they build.
PC parts are made according to universal specifications so it's not hard at all. I would even venture to say that it's less building and more setting up. I mean it's not like you have to solder parts together or anything like that.