I think those two are thinking about CO not CO2 but if your burner is messed up it will put off CO so its not entirely a bad idea.

As far as doing hydro without an EC or PPM meter.. the best I can say is one day your luck will run out..

Where do I find the cheapest ppm controler, and how much is it, and do I want to buy the cheapest one?

eb-ay $35 telair ventostat.. maybe another $5 for the cable so you can change the setpoint from factory 1100ppm to your desired ppm level