Re: adding a buzzer to comply with regulations — could be, although I expect that there are a lot more specific regulations that you have to meet. A cheap buzzer/speaker is an interesting idea though.
Re: calibration button, I think that makes sense, and a button is an easy add.
Re: display, I think the LCD display would make it a lot more expensive and cumbersome. My suggestion would be to have a set of RGB LEDs, one for each sensor value, and have some smart defaults for color range (green = low, red = high), but which can be modified by the user.
Re: MQ-7 — looks like a reasonable sensor to include, it’s not crazy expensive like some of the other MQ sensors. I’ll do some research into this sensor and the one I had spec’ed above for carbon monoxide to see which one looks better on paper.