As far as the HPS, I'd like to know where you get one for $20 yes you can get the bulb for that much, but you cant just plug it in, you need a ballast.
You can get a 150w HPS Home security light and take it apart and rewire it remote ballast and use that... thats what I do. Cost about $90.
As far as Ventilation get you some computer fans, and dryer vent tubes. Cut holes install one computer fan to exhuast air and one to blow air in. I get 120mm ones at radioshack for like $9 a piece and wires them up to a 12v charger I also buy from radioshack. add the Dryer vent tubes so no light can get in through the fans.