Hello @jevonsp,
Welcome to the forum!
As all other said - it’s better to build something only if it’s needed for you/your company/family, then you may offset costs of having the needed thing by running storagenodes.
If you think about to buy anything only for Storj - then you may not have a ROI anytime soon, it may take years. This is because the shared storage is used by real customers to store and retrieve their data, not some generated digital trash. Thus the usage is not predictable and it’s hard to calculate a ROI.
In terms of HW, it is better to use energy efficient systems, preferably refurbished/used to improve sustainability, but the basic requirements will depend on your needs. The node can work even on a router: Running node on OpenWRT router?
Regarding OS/software, I would agree with @arrogantrabbit, TrueNAS is a better solution than any other. It even has applications for backups to Storj bucket and to run a Storj node. Otherwise you may use any other software, even pure Ubuntu and configure everything yourself.