USB hard disk? Especially a powered dock, 2tb used is only around $20. Plenty of space for the blockchain.
Used HDD? I wouldn't use it, especially to run full node & leave it 24/7, who knows the remaining life of the HDD.
Wouldnt an atomic pi which was available a month ago be more efficient since they run on an atom x86 architecture and cheaper then a pi?
Then Raspberry Pi 0 is better option, it's only $5 & still good enough to run full node.
FYI, Raspberry Pi focused on cheap barebone PC & open-source hardware rather than performance