This was my biggest issue and decision. I was told a million things by a million people. So here's what I did...your space is a couple feet higher than mine, but shouldn't be much of a difference. Check out
www.4hydro.com. Awesome awesome site, and they hook you up with discounts based on how much you order. I got my fan, my plugs, and my carbon filter, shipping paid and like 15 bucks off. Anyway, I got a sunleaves windtunnel 4in 200cfm fan. I was running it through my air cooled reflector, but 400w doesn't put out near as much heat as you think or people tell you it will (at least I haven't been above 80 degrees with the fan off for 15-30 min). I have my dr120 set up so that one of the flaps is half open and fresh air comes in from the window that's only about 2.5 feet away, so I don't even have to run it all the time. It may sound like not the best idea, but the kids are BLOWING UP and have no issues so far.
Big issue I kept hearing is "you need two fans". I'd like to call SHENANIGANS on this if you have a space as small as mine. This is just based on my few weeks experience, and it might change once I get bigger lamps/bigger area to work with. You're running 400w like me, so one will do for now, even one as small as mine. You can get a can fan that pulls about 170 cfm through, but not for the 125 bucks I got the sunleaves windtunnel on. Only drawback is that you're supposed to mount it to a wall or something. I didn't do that. I just strapped it up to my cross bars up top and fed the ducting right out the side. It works awesomely. The one thing I would recommend is 4in duct clamps. I was using duct tape, but having to redo it about every week. haven't had to touch it in 3 weeks since I got those clamps.
Hope that helps you out man. Good luck with the flowering! I'm hoping to start mine in a week or two!
