You are wayyyy off base here. You're a new grower trying to start with a 400w HID with no proper ventilation?
Why are people telling you to put your 400w HID right down on seedlings? Go out and get yourself some CFL's. Just get like 1 27w per plant you are growing.
Put those above your plants until they have a few leafsets. Once they have matured a little and can handle being BLASTED by the light go ahead and upgrade them to the one light. But you have to find a way to keep that room cool.
I know you think this is going to be easy, but it seems to me you have a very limited chance of success.
You're going to have odor problems due to poor ventilation, poor filtration (can't have one without the other), heat problems, light leaks, etc. Should focus more on getting and ample grow room setup BEFORE focusing so much on the plants themselves. It helps ensure success which is what you want.
Good Luck and happy growing, make sure to check out the GrowFAQ on here, couldn't be more useful.