I suspect the MH-Z14A would be the easiest. One of its output modes includes analog, which would hook up nicely to the ADC (ADS1015) on Enviro+
For best results you would have to interleave gas sensor readings with your CO2 meter, since you would ideally want to set the ADS1015 gain to 2.048v instead of 6.148 to take your reading with the most sensitivity.
You can see how the gas sensor reads the ADS1015 here: https://github.com/pimoroni/enviroplus-python/blob/master/library/enviroplus/gas.py
Depending on your use case, you might even want to hack CO2 readings right into here. I mean… CO2 is a gas!
Actually I should probably add two features into this gas.py:
read_other() # Read 'in3/gnd'
set_other_gain() # Specify the gain to use when reading the additional analog input
And then I think I’d have you covered.