What's the best RasPi to use to make a magic mirror at the moment and why?
Actually you can get an RPI4 for
$35 US 2 gig
$ 55 US 4 gig
$75 US 8 gig
or an RPI3
Model A 
Model B [First model]
Model B+ [Final model]
They are also working on a 64 bit variant of Raspberr Pi Operating system.
There’s also a Raspberry PI beta that will allow booting from the USB slot.
Any of those would work for your mirror.
@videogame95 - I normally put a travel power strip inside the frame for the mirror. I’ve since started adding external switches that cut the power to the power strip, but you can get by with just using the power strip’s plug.
Once you have the power strip in the frame, you can plug in the 3.1 amp adapter into the strip, along with the monitor, LED supply or whatever else you have back there. With some careful shopping, you might find a strip with a USB C plug built into it.