Here is my 2 cents.

Everything you spec’d will work fine. As for the monitor, just about anything will work. There really isn’t a “best” monitor.

What do you want to display and where do you want to display it? Quite often builders will get a monitor that is 1/4 the size of the mirror and mount the monitor so they are displayed in the upper left or right of the actual mirror. Is this to be installed as a bathroom mirror? Do you want to stare at the weather report in front of your face while you are trying to shave?

As for the best “quality” monitor, anything you buy will work great. Figure out the size you want then look at how the thing is rated by customers and professionals. People here are from all over the world. Many in the U.S. use Craigs List to find monitors.

Don’t install pm2 until you are comfortable that all of your modules are working correctly. There is good info that can be had from starting the mirror from a terminal session.

Buy the Raspberry Pi, find an old monitor laying around and start playing. The monitor doesn’t even have to be HDMI. You can get an adapter to go from VGA to HDMI . https://www.amazon.com/gp/slredirect/picassoRedirect.html/ref=pa_sp_atf_electronics_sr_pg1_2?ie=UTF8&adId=A1023898KTBOLVX626S3&url=https%3A%2F%2Fwww.amazon.com%2FActive-Adapter-Benfei-Converter-Gold-Plated%2Fdp%2FB01KLKQN9U%2Fref%3Dsr_1_2%3Fs%3Delectronics%26ie%3DUTF8%26qid%3D1483978418%26sr%3D1-2-spons%26keywords%3Dhdmi%2Badapter%26psc%3D1&qualifier=1483978418&id=4118648475855561&widgetName=sp_atf
You don’t even need a keyboard or mouse. You can VNC into the Pi.

In my opinion you are over thinking this. Decide how big you want the mirror and frame to be based on where it will be placed then worry about the monitor size.

PIR sensor help please