CO2 can be used the entire time. We started using it in only the flowering room, due to the costs. BIG improvement.
Doing something, and doing something RIGHT the first time are totally different. Why do something only to have to rip it out and do it all over?
Personally, I'd rather wait till funds allow me to do it right. Might be good advice to follow if you think along the same lines.
I'd rather not end up with moldy bud, or lose my crop.