My solution to this is setup an SFTPgo drive or mount a restic drive and have docspell (alt to paperless with worse UI) to watch for changes.
Main advantage of docspell that it consumes files without being destructive to consume folder. Having a network or cloud drive was already the way I stored my files, so I did not need to change much.
Where are you buying OptiPlex’s for 50? Get one for 30.
I would say look into i3 (more power efficient) and get some cheap budget SSDs or dirt cheap HDD (it will be plenty fast).
Because win11 requires TPMs there businesses had to replace office drone email terminals, so there are plates of those getting scrapped. Maybe less, so most probably have been turned to shredded metals.