Just depends on what you are trying to accomplish with the server. If you want the server to batch download emails while driving down the road, for example, you can do that.
If all you want is storage, then just get storage. NAS is basically a server that only does File Services. Most of them use a slim down version of Linux and use SAMBA (smb) to share out the storage.
If you wanted a custom and cheap NAS, you could start with a Raspberry (small computer) and add SSD's or what ever, but you'll need to know a little bit about linux OS.