Ok, if that's the situation than I would go with a 400. Reason being that when you start seeing nice buds you are going to get the grow itch big time and want to expand. With a nice 400 hood you can mount your 400 on a light rail and have and have solid light over about 8 foot. Your going to want to have some space on each end to mount plugs and put your fans and dehumidifier. I really like the idea of crawl space vents. I would mount two GOOD 8" inline booster fans in the floor. One on each end, one blowing in the other out. Hook them to a thermo stat and be be done with it. By good booster fan I mean the hydrofarm one, all the other booster fans suck ass, but this one actually throws around 300 cfm.
http://www.amazon.com/gp/aw/d.html/ref=redir_mdp_mobile/192-7687516-7503254?a=B001G6XSGU
very nice flow pretty quiet. Use nets on the fans so you keep bugs out of your room. That is the last thing you want in there!