It sounds like you really just want a nice storage closet with a simple closet to child proof/nosy neighbor proof it.
If thats all you want then put a decent lock on it and fix out the inside as desired. Its not going to stop any sort of determined thief, but that doesn't appear to be your intent in this.
You can get simple felt at Hobby Lobby to felt shelves. Id' consider a humidifier depending on your exact location, and a nice interior light. Else a new lock and it should meet the need of what you want it for.