-Are Tamiya tanks metal or plastic? I’m getting conflicting info on this.
The three Tamiya tanks you've cited are a hybrid of plastic and metal. Metal is used for structural parts such as hull bottom, torsion bar suspension, suspension arms, etc. In this case all three of them have plastic tracks, with the King Tiger's being particularly weak.
-Is there an advantage to plastic over metal?
As pointed out above, plastic is easier to work on. Molded plastic parts have finer detail than molded metal parts. Plastic is also lighter, which taxes your electronics, drivetrain and suspension less than a heavy tank with a lot of metal parts.
-Are Tamiya tanks only available as kits?
They are available as XB (Expert Built) tanks, but I believe you would need to special order them and they don't come cheap, probably 2-3 times the cost of the unbuilt kit although it comes with a radio controller and battery.
-Are parts readily available for both Torro/Taigen and Tamiya tanks?