Good case, average PSU.
As already pointed by @dan, raidz2, implying four drives to begin, would be safer.
This I don’t like at all…
And, if you’re buying parts to build the NAS, it is a pity you’re not going for ECC.
By default drives would be kept spinning. You may try to have them spin down when not in use, but plan carefully, as repeatedly spinning up and down is what most stresses drives; a few cycles a day is fine.
Re-encoding at a lower resolution if the client cannot do it by itself. You may well not need it.