I built my OMV box from an old i7 PC with Asrock H81m-dgs r2.0 motherboard in an old atx case with a new PSU , 16gb ram and an i7 processor.
Boot hard disk is a new WD Green 121gb SSD, so it boots quickly.
Data drive is a new WD 4Tb sata drve for the NAS shares
A repurposed Seagate 1Tb sata drive for my subversion repository
a 14" second hand LCD (cost me $10)
a recycled keyboard and mouse
OMV Stoneburner idled away on this since stoneburner came out, then i recently pulled out the old hard drive w OMV2, rebuilt with new drives and OMV5 and re-synced the data across to the new server.
CPU idles away at an average of 0%, ram useage with svn at about 10% and a load average bubbling along between 0 & 0.11, no wait, 0 & 0.17.
So an old machine could, with sufficient attention to backup of your NAS build and regular data backups, rumble along quite nicely without too much effort and virtually no load. Of course when I add more docker services it will utilise more of the load capacity, but at the moment, i compare the current load on the system to the amount of room one person standing in the middle of an empty football stadium takes up!