I’ve had a great experience with the TrueNAS Mini-X system I bought. ZFS has great raid options, and TrueNAS makes managing a system really easy. You can get a box built & configured by them, with 16 GB ECC RAM and five (empty) drive bays, for about $1150 at the most affordable end. https://www.truenas.com/truenas-mini/
One thing to be careful about: you can’t add drives to a ZFS vdev once it’s been created, but you can add new vdevs to an existing pool. So, you can start with two mirrored drives, then add another two mirrored drives to that pool later.
(A vdev is a sub-unit of a ZFS storage pool, and you have to choose your RAID topology for each vdev and then compose those into a storage pool)