Just for the record, I think you’re addressing the wrong audience here.
Even though server-based software is available from our repositories, Manjaro as a distribution is geared more toward desktop and laptop use, and furthermore, it is a rolling-release distribution, which is commonly not a type of system suited for deployment on servers — too many updates, and too dynamic.
Therefore, I doubt that there would be many people here who’ve got much experience with the kind of hardware you’re talking about, and given that you’re mostly inquiring about hardware solutions, I recommend that you direct your inquiry at a hardware-based forum.
As for the redundant PSUs, I do have a little bit of experience with that, but what needs to be emphasized here is that you must make sure that either of the two PSU channels supplies enough juice to your motherboard and drives, because those things will commonly be advertised with numbers reflecting only the total Wattage that the PSU supplies, but what matters is how many Watts the unit can continue to supply when one channel drops away.
Other than that, make sure your machines are connected to uninterruptible power supplies, preferably with a generator for backup — professional data centers do that too — and of course, that you have your drives in a hardware RAID setup — RAID 1 at minimum, but preferably RAID 10 (which is the best approach) — and hot-swappable.
For backups, I recommend HDDs instead of tapes. HDDs are faster and have a longer data life than tape, but they’re cheaper per unit of storage than SSDs, and you don’t need the speed of an SSD for storing backups only. And of course, they should be RAID and hot-swappable too — RAID 6 is probably the best solution there, because it offers double redundancy.
Software-wise, it might be a good idea to use virtual machines — maybe on top of Xen — so that you can migrate a running virtual machine to a physically different machine over the network, and so that you can run a watchdog to fire up a redundant virtual machine if the primary one goes offline for whatever reason.
That’s about all the advice I can give you, because again, Manjaro is not a server-oriented distribution.