IMO Co2 is not worth it. You'll need a regulator with an electric solenoid for the tank which will run you between $100 and $200. Then the tanks are about $60 to $100. fills are about $16 dollars where I am. You can also rent a tank from a welding shop but then you have to give ID (Don't know if your the paranoid type but just thought I would add that) an in some cases sign a contract to rent for a certain period of time. Also your room would have to be air tight are there would be no point. You could also look into the tablets, homemade setups with yeast, or theres even a bucket that costs about $100 that you can use for a whole bloom cycle. In my experience is not worth it, I have the whole setup and bought my own tank and I no longer use it. But yeah to answer you other question you can start using it at any point