I think you'll do fine with a 400 HPS as they do not put out that much heat, at least nothing that a small AC can't handle. You'll have to do some construction for the ventilation system. Remember your babies need fresh air as well as exhaust, 2 inches in most cases will not work. Also take into consideration future upgrades if this cycle goes well.

I know it's a basement but don't you have a window or something? I wouldn't use the house ventilation or vacuum system in conjunction with your grow, isolate the two so there is zero chance of odor escaping.

What are your relative humidity levels in the basement?:rastasmoke: