I grow in an 8 x 12 x 8 foot tall shed, outdoors, in the southwest desert.
Summertime temps are topping 110 right now, winter temps sink to...below freezing.

No way I could do this without a wall-mount A/C for the summer (11,000 BTU from Walmart - $250.00) and a floor heater in the winter. I use an oil-heating radiator (DeLonghi) so the heating element doesn't come on (glow) all night, stressing the ladies. Also, in the winter, I set a fan directly above the grow, and aim the airflow down. This blows any heat back down on the plants.

Other than the A/C in the summer, I don't use an intake fan. (unnecessary)

Are you going soil or hydro? If going hydro, make damn sure you can control res temps 24/7/365. (I grow in soil)

Heat is easier to deal with than the cold. Check the "Dealing With Heat Issues" thread. The link is in my signature.

I personally don't mess with the CO2. Have never had a solid reason to use it since I don't grow for quantity, I grow for quality. And haven't you heard...CO2 emissions are killing our planet...? (LMAO)