One famous choice is get the HP N40L and install Xpenology.
Great performance and flexible, N40L is fully stable on Xpenology too.

There's a NAS thread on another section anyway - https://forum.lowyat.net/topic/2931469

Prebuild NAS are expensive eg.Synology but considering the fact that the software have been tested to be fully stable on the hardware compared to self build which you may face a few hiccups if it is not configured properly.