I suggest that regardless of what or when you find for a new furnace that you at least have a CO2 detector on each floor of the house. some are integral to smoke detectors while some other wall mounted units have a digital readout of ppm which are very helpful.

I advise against installing your own unit unless you really know what you are doing with codes pertaining to gas line and venting. As an electrician I see shoddy DIYS electrical often.