Maybe this is the blind leading the blind, but I wonder what kind of quantity do you want in what kind of time frame?
If you want something in 3-4 months then that is probably just fine for you. If you want to let the plant grow and decide when then I'd suggest 5 or 6 ft depending on your strand. You can gain up to 12" or more on flowering if your using any blue with the red spectrum. Also, I'd suggest ordering out and getting a bit higher wattage bulbs, 40-60 watt, 'specially for flowering. That or add a few bulbs lower dow so the whole plant can get light and do well in production. With ventilation I've used two 120mm fans rated for 70 or 80 CFM. Wired them to AC to DC converter rated for pretty much what the fan ran on. Worked like a charm. I put filter over the intake and exhaust. (Intake for the fresh air and light and the exhaust to help keep down on light emmision.) Just goto Walmart, over by the air filters and fans, and grab the active carbon replacement filters and cut them in half. Either duct tape down or you could construct a hold down ring to screw down.
There are a few more variables then the simple ones.
