Absolutely most critical thing is covering up exposed skin.
When it's that cold, I'd add an active synthetic insulating layering (usually the TNF Ventrix these days). In the past I tried down puffies, but I hate down when I'm active... super bulky and loses insulation properties when wet.
I usually don't insulate my legs, but have some old Burton "expedition" weight fleeces I have lying around.
I also normally wear lightweight impact shorts to protect my tailbone, but during cold snaps it has the added benefit of putting a non-compressing insulation layer between me and the chair.
Let me know if you roll through the City. I can't remember how much bigger than you I am, but if it's a close fit I have some extra stuff lying around that's been hanging around in my closet while I'm on the sidelines.