About all you need is the open_cl package, a small number of recent vintage supported GPU cards, small HDD’s (enough for the OS and some swap space) as much ram as you can stuff in each machine and a good network card and switch combo. The reason for the GPU’s? Bitcoin mining, password cracking/recovery and any other project that supports using multiple gpu cards. If your machines aren’t up to the task, start with a PXE booted cluster of RaspberryPi units (start with an odd number like 3 (one as the master control unit). I had plans to do this myself, but my fixed income here just isn’t up to the task of doing more than paying for the basic bills.
From the Central Offices of the Technomage Guild, Open Cluster Development and research Dept.