I have a Patagonia nano puff and HW compressor for mid layers if it is really cold along with a UA cold gear ninja mask. As for pants I have some 686 cargos that have fleece liners for when it is really cold along with Burton and Arcteryx gore pants. I run kind of warm so I am unzipping for the run down the zipping back up for the ride up. The biggest question for you will be what gets cold first on you when it is -15c (5 F) for me it is my face so I just make sure I am really covered up.
I don’t love insulated pants cause once it is warm out they suck to wear but they do work great when it is really cold.