Well sounds like u did alot of thinking about this...only one way to find out....but i dont think u need the C02 bro. It wont hurt, but as far as the the cost..ur not growing enough plants to really justify it. I run 4-8 lighterz all the time and never needed it...if u look at my Lavender grow on here u will see 4 1000w HID's with no c02 and this will produce 5-6lbs. If u have the extra cash and time go ahead...but not really needed. Also u have to pay super attention to detail with c02 and adjust almost every setting, time, temp etc accordingly.

Anyhow...love to see pics when u turn the lights on!