My setup, which I need to finish, is an old laptop.
You can get a SATA hard drive adapter for the optical slot on many laptops. So you can put in two 2TB hard drives. My laptop only has USB 2.0, so that's a downside. But you have a built-in UPS (the battery), and my battery is a OEM high-capacity one which still works OK.
So the only new parts are the adapter (< $10 USD) and the SATA 2.5in hard drives. Just be aware of the height restrictions, some internal hard drive cavities on laptops can only take the 7mm or 9.5mm high drives, and not the 15mm drives that can be used on a NUC.
My old laptop is a quad-core AMD (which is OK-ish) and has 8GB of RAM, so it will be running some other stuff as well.
