Just beware of too old hardware that may end up costing more than expected because of their power draw. A NAS usually stays up 24/7 and power saving and reliability become important. I would also avoid anything with a fan, particularly if they expect it to be running constantly.
A mini PC with enough internal space to host a long m.2 SATA adapter (0) can become a quite compact solution; many used low performance Celeron based systems considered obsolete for desktop stuff would be more than enough for that task.
0: https://www.ebay.com/itm/256268302567
(no relation with the seller, just found searching "6xsata m.2" and removing those with lower feedback ratings). There are also 5xsata similar cards.