overall looks fine to me
somethings to note
make sure whatever model you end up purchasing are cmr drives and not smr drives.
i would also run a smart test on the drives and check how many reallocated sectors there are before you commit.
people tend to buy drive cages (that could even be designed for other cases) and insert them into the case you already have.
looks sufficient to me but i would very much recommend upgrading the ram, hexos is an operating system that really likes ram.
theoretically you don't need your storage to get through setup
but there is almost nothing you can do with hexos without storage drives.
At most you can just check if you are able to instal the os
as for your sata power question
it is out of my area of expertise