Are you planning to use a single board computer (SBC) like Raspberry Pi ? If yes, you have many options on which software to use on it. For starters, Ubuntu officially supports Raspberry Pi. That is good, right ? Since you might have used Ubuntu on your desktop you might like it on your server / NAS as well.
DietPi is a simple and lightweight software solution which will work on may single board computers. You can find the full list of boards on the link above.
I am not sure how good they are but they definitely worth a try from what I heard. If you run into any issues, they have a forum under the same domain where you might find some community help.