FYI, Plastics are harder to come by now and more of the boot manufacturers are making synthetics. From Scarpas Phantom Range to La Sportiva Batura and upwards. Plastics are kind of old school.
Maybe check out the Scarpa and La Sportiva (then Boreal, Raichle, Salewa,Zamberlan) websites, they can give a semblance of information about what boots are designed for.
In a gross simplification, the more expensive the boot, the colder temperatures it can be used and generally stiffer.
And as you go warmer and stiffer, you get less comfortable for walking.
As for double boots for cold. The double (or triple) nature of the boot means better insulation plus the inners can be taken out and dried (or worn in sleeping bags/tents ). Its really hard to dry or warm leather or single boots.