You can replace the power supply with a discarded USB charger from that old mobile phone of yours/relative/neighbor/etc OR discarded computer power supply, and run a hundred of these off the one.
For the first 6 months or so, I’d say that few GB will be all that is needed for an average vault. Remember it is not the amount of storage offered, but the amount used that earns. So initially one expects the network to be small, thus each vault has only a small amount in them.
(Janitor, who in their right mind is going to spend all their coin on day 1 of network being live. What a joke. Unproven system and all)
I would prefer the 9$ ($8 on monday) C.H.I.P computer available next year, that has wifi built in, enough SSD for a starter vault and draws similar power to the $5 Pi