Well,I would figure this would be a good SCP to add ingame as it can turn into anything. You can read the full info as what it does here -> http://www.scp-wiki.net/scp-157 <-

It is not hard to model but may be hard to code,I don't know. As I would recommend what it if useful for in the game,It can change into a gas mask,eyedrops,ballistic vest even other people.

It wouldn't be hard to program, and you probably wouldn't even have to model the SCP itself but only the containment area/box.

You could use an existing model that is useful in the game, like have what looks like eyedrops or gasmask in the box, and when the player uses the item nothing happens except later on you die.

Well,It could be that you use then and about 30 seconds later you will see a text at the bottom that says "You feel a stinging pain" then another 10 seconds later you will scream (I guess there can be a scream.ogg or something like that).