Earlier quoted context omitted.
What CO detectors would you recommend?
Any off the shelf product should be fine (there are standards they have to meet). Kiddie and First Alert are both well regarded brands. I use First Alert personally (they have model with Z-Wave integration that ties into my home automation system). We carry a few Kiddie smoke/CO detectors on the fire truck to install if we notice a home doesn't have one (or if the one they had failed).
For those who haven't seen it, here: