Cheap gaming PC for Diablo 3/Starcraft 2

I don't play much games, but the only two games I do play on rare occasions are Starcraft 2 and Diablo 3. I mostly use my "computer" (2013 MacBook Pro Retina 15") for editing photos/videos and watching movies. Recently my laptop's been on the verge of death, constant overheating, battery not holding charge etc.

I am currently using a 27" thunderbolt display, but am thinking about purchasing a 34" Ultrawide monitor.

I have a very limited knowledge in building a PC, and no knowledge in what parts are better than others.

I don't have a set budget, but seeing as how I don't "game" intensely, I am hoping for a relatively cheap build. Seeing as Starcraft 2 has been out for 5 years, I'll be okay with slightly outdated hardware.