I would suggest buying one that somebody has already made. This is because the price will be lower than if you had to build it but the only set back is trying to find a decent one near where you live.
You can go with an AMD APU, which are about $100 or a bit more, and have 512mb of video ram. You can get decent gameplay one medium settings. But I recomend you to get an aftermarket cooler, like Noctua. 4Gb ram. 500Gb HDD, a cheap case which has at least 1 exhaust fan. You might get something. Good luck.