What needs to be considered when building a winter bike?
Good for you, to think of riding as a year-round activity.

I would choose an older steel frame (Specialized Rockhopper from 1994?). Steel is very durable, and the older frame is less to be worried about with salt. Put a little FrameSaver on the insides, and don't worry about it. (I'm not an expert on these, just a Grant Petersen fanboy).

You really don't want to get stranded in an Ottawa winter. Add some storage, and carry:


a plentiful toolkit, including a multitool, pump, and spare tube. 
cash for bus &amp; phone
spare batteries for your light
compact warm items, like a balaclava, gloves, and a wool hat


The trick to rim brakes in the wet is to dry them out before you need them. When you anticipate a need to stop (approaching a driveway or intersection, for example), lightly apply the brakes. After one rotation of the wheels you'll have most of your braking power back. After a second rotation, they'll be in great shape.