You could consider picking up an old refrigerator for next to nothing (or less) and using trays of dessicant instead of a dehumidifier. If the door gasket is decent, it will be light-tight, and the interior finish lends itself to easy cleanup. For hanging paper, remove all of the shelves but the top one, or install your own hanger bars. For flat storage, use the existing shelves or add your own---with the refrigerant removed and the power cord cut off, you can safely drill into the cabinet to mount things.

If the interior is going to be crowded (we can only hope!) the easy, safe way to get air circulation is to use a 12 VDC electronics fan powered by an AC adapter located outside the reefer. In a closed environment, it won't take much to keep the air stirred up.

Incidentally, if you want to "condition" paper, there are various salts which, when in contact as both solid and saturated liquid, will maintain a constant humidity level until either the solid has dissolved in captured water or the liquid has given up its water to the environment. Much simpler and cheaper than a humidifier/dehumidifier setup with a control loop.