If you're not opposed to it coming with Linux, all data says that the Steam Machine is a good deal for its specs. It'll even have the added bonus of feeling familiar while still being a PC.

If you really wanna build one yourself, you can always check Newegg for parts. I think they're going to be having their bit Black Friday sale going on soon too.

Otherwise, shop around for pre-builts! You might be able to get some at a good price, especially if it's a model that's on clearance.