Usually buying an old dell tower works great for what you are trying to do. Buy an Optiplex 7010 MT (the big/fat one). I've used this several times and it tends to run cheap. It supports up to an i7-3700K, 32 GB of RAM at 1333 MHz, and you can use cards up to 211mm in length, I was able to cram a zotac 1080 ti mini into it for example. Normal sized power supplies WILL fit, but you'll have to remove the motherboard to get it in it's slot, or pull up on the plastic latches for the side panel. For example, I got an optiplex 7010 for approx. $50 on eBay. Which came with parts inside it too. A pentium of sorts for CPU, 8 GB of RAM typically, a HDD. It's really the best deal out there.