Use a regular PC and pop FreeNAS onto it.
Now that I've had a think about it, I've upgraded my NAS more often than my PC in the past 10 years.
The main reason for this is that the earlier NAS's have limited support for large capacity drives, so I've needed to upgrade.
ZFS can upgrade your storage pool in place and grow the volumes as you replace drives with bigger capacity ones
Only if the firmware supports it :)
My last NAS was indeed a FreeNAS box running NFS.