Buying a new carbon monoxide detector. What should I look for?

[Tap]

Are there certain features that are beneficial? Is it better to go less expensive but have multiple?  Our house has 7 wired in smoke detectors but no wired in CO detector. We have a plug in C0 in the upstairs hall, (outside the bedrooms) but I am sure it is ++well over+++ 12 years old and could probably be retired. 

We have a two story house. Our water heater is gas and is in the garage on the main floor ( which has some unrestricted fresh air vents).  The bedrooms are all upstairs. Just over 2200sqft. 

Any suggestions or information I need to know? 

[mom2scouts]

I'm glad you asked this. I have ancient plug in CO detectors and want to replace them, but I don't know if there's any important new features either.