The minimum requirements are an i5 processor, 4GB RAM and a GeForce GTX 550 or Radeon™ HD 6XXX or better. Also required is 64 bit Win 7 SP1 or later, but Win 10 (64 bit) will be on any new Windows PC. I've seen i3 CPUs with hyperthreading mentioned as running the game fine.
The recommended specs are an i7 CPU (i5 should be fine, though), 8GB RAM (a good idea, and RAM is cheap) and GeForce GTX 770 or AMD R9 280.
I have not built a computer or upgraded recently, so am not sure what the best bang for the buck is for systems or various components.
Check out
Tom's Hardware; that site has GPU ranking charts and has a best performance/price list, etc, and the last time I looked the forum there had topics with purchase advice. Searching for 'best cheap gaming PC', etc, should give you something to start with (if using Google, click on Tools, then change 'Any time' to 'Past Year' or custom range and set it to the past 6 months, or something).