I am about to go for Dell Optiplex 3020's. They meet the min system requirements with onboard Intel HD Graphics. They can easily be upgraded with a GPU if you get the MT or SFF form factors. And you can easily get them for under $100. They are being phased out in masses and you can get from just about any site. Buying the MB/CPU/Ram individually you can get down even cheaper in $$.

It's nothing fancy, but who needs fancy to run 4 side machines networked with a main rig.