Hey JFJ,

1. Either would work fine.

2. Yep 400 hps is great for that space.

3. You're complicating matters mate, you only need one fan, you have the lot attatched to it. I have mine set up like this, tho there are other ways. (i'll use ------ to represent ducting)

Carbon filter-------Cooled reflector--------Fan

see the pic below, you can't see the fan coz thats outside the tent.

4. Sorry i can't say, a lot of it depends on the temp of the air you're sucking into the space.

Good luck