AC for sure!
CO2 is the last item to add.
CO2 is used as a band-aide fix for high temps. Higher temps make better use of the CO2, but temps over 90F hurt the THC. The higher temps open the stomatas (pores) in the leave so they 'breathe' better.
Getting temps down and controlling humidity is/was my biggest improvement and the plants showed it. (higher temps can lead to lighter/fluffier buds too)
Not sure of your setup, but mine was a closet, sealed air cooled hood, and temps stayed pretty good. Going for the gold (temps) next time I grow. THen I'll toss the CO2 on.
Oh, if you are thinking of a CO2 generator, they will add more heat and raise the humidity levels. (also keep it closed loop control)