A NAS is more than a disk, an enclosure, and an ethernet port: it includes the software to handle all of this (for instance, the handling of permissions, access lists, passwords, and so on) and the hardware that goes with that software.
So what you need to buy is a diskless NAS; you will find many just about anywhere, online or in a store. Just make sure the model you chose can host two, 2.55in disks: some only host a single disk, and some only 3.55in disks. But that's all.