You did good on the cabinet. Now you need to find out how much light leakage it has. As far as reflective material, Wally Mart has some safety blankets that many people have had success with. They have a reflective material on them and you can place them in your cabnet and they may also help with the light leakage. They are only a few bucks. You can also go with foil using the dul side to reflect the light, the shiney side can make for hot spots.
For lighting, if you go with a 400w you are going to open up a can of worms in regards to heat. I would look into a 250w hps, or an mh/hps conversion set up. For some cheep and also higher end lights I would check out, www.htgsupply.com
As far as the air vent aperattous you have, I don't know much about that.
hope that this helps some.
Adieu