Well, for the transparency part, you could just use backwards face culling, and make the front side opaque.
and for the collision part, you could make the model have no collision bounds at first, until you get to the other side which has an invisible object so where that when you collide with it, it makes the door object have bounds, so you can't go through.
My Mind is very odd, it can memorize hundreds of commands, memorize 2 dictionary's worth of words, and make detailed 3D models. Yet my mind has trouble with the most simple calculations.