You can easily get commercial grade metal security doors through your local home center. They are basically metal doors with reinforced jams.
Door opening is to small for any pre-fad doors .
If it's really just to keep the honest people out your on the right track . They make hinges that the pin does not come out . You see these more on exterior doors that open out . I assume the door opens out so you might want to replace the door stop trim with something thicker like 5/8" or 3/4" and use nails with heads rather then finish nails . That would help keep it from being kicked in .
As for the interior I would not do much at first cus your bound to change things up a little as you start to use the space ( add , move or remove shelves ) you can always add but it can be a pain to remove .
You need to make sure that area/space will stay dry . That type of floor and walls can leach moister through them .