NAS drives are better (with the possible exception of WD Red). Make absolutely sure you are buying CMR and not SMR drives. SMR and ZFS do not mix. Do not buy WD Green or similar - they aren't really designed for this.
Oh dear, all my drives are WD Red. Are they that bad?
So it sounds like I'll just use my onboard 6 SATA ports with 8gb disks, and make sure I don't connect to the Marvel controller. Swap to a bigger case. This should last for a few years and then I can add to the pool with a new vdev.