Even Windows 10 has advanced file sharing. If its RAID and less than 20-30 concurrent connections to the machine, I'm not sure you would have an issue running a client OS for this. Obviously you would want to remove the footprint by blocking lots of ports on the firewall and just using it to store files. I don't recommend going this way but if you can't upgrade to Server 2016 and don't want it anymore you can always give this option a try. If you really want to reduce overhead you could run a cloud-based storage option like Dropbox, Microsoft OneDrive, Google Drive (Backup & Sync), etc. Depends on what permissions are required on these folders and who should be able to access them.