I'd suggest the 183 for pow days since they offer a bit more stability for and aft compared to the 173 when things get deep or chopped up. The Shiros ski quite short, and are very easy for their width. Use whatever bindings you are accustomed to, like and trust.
About the same as the brakes in your car. DOT5 fluid works perfectly fine at -20F.
The oxygen overshoe is a good option for cool weather riding, and these offer great wind protection. The Large fits my size 42 sidi shoes well. We'll see how they hold up to extended use.
The newest T-Race is again a powerful, stiff boot for driving big skis with powerful, active bindings. It's definitely more versatile for touring with added range of motion in the cuff than the previous candycane (red/white T-race model, 06-09). The new red T-Race has better buckles and improved power strap, but I did prefer the classic (overlap-style) precision high liner to the new Intuition (tongue-style) speed pro liner.
The old candycanes are stiffer than the 10/11 red/black model. Scarpa claims their powerblock tour mechanism provides upper cuff flex in tour mode, but locks the cuff for downhill ripping. Even when locked down in ski mode, the powerblock allows some play and more thus more flex in the upper cuff. This essentially makes the newest T-Tace the same as the newest T-1 (if you're trying to decide between the two). The pebax plastic in the newest T-Race seems softer than the candycane model.
If you're upgrading from candycanes and want similar stiffness, swap your old red candycane (non-walk mode) post with the new powerblock tour modes. (see pic showing the result)
It's quite easy to do at home, or you can go see a professional bootfitter if you don't want the fuss. Do some googling on DIY baking liner videos, instructions.