for your filter you are going to need a pretty decent fan. I don't think a computer fan will do it. and yes the 6" filter is fine, it's what i'm using for my cabinet.

a cool tube will eliminate heat issues for HID lights. I really recommend them over fluorescent--you get better, faster growth with HID, and most importantly, more bud. I know they might seem expensive, but think of it as an investment that pays you back in weed.