Key locker is quite full, lock needs to be replaced, and generally does not leave an audit trail.
Larger non-electronic key locker offer was 150€. Electronical ones have low key count and are in the k€ price range
Instead, what about a package locker? Few ones for zoned keys (lockers, vending machine, mailbox, misc) + general use lockers?
To distribute the cost of Pi-keycardReader-buttons, 16 seems like the sweet spot.
Lockers can be like package lockers, or drawers (SmartPost XS), or flat doors on the side (keys). Access to controller should be with a normal (tubular) lock. The locks have mechanical backups, which could be accessd from behind, if the back comes off.
Existing door controller software could be reused for the lockers. The majority of work (and material cost, 150-200€) would consist of building the structure itself. Consider buying/reusing a pre-made structure.
The locks should be between/outside the lockers themselves, as access to the wiring could be used to bypass door open/close, and forge the audit trail.
Electronics total ~150€