As with anything, there are lots of options. Some shoe covers are just wind blockers, others have waterproofing and insulation.

One thing I will say - especially with you being diabetic - is to make sure that your toes aren't constricted. My feet actually stay warmer with less on them, than they do with a lot of insulation that takes up room where my feet are supposed to be. Toe covers are right out - on me, they completely cut off the circulation to my toes.

I have a pair of the Voler shoe covers (light wind blockers), and I like the fabric, but the zippers broke within a year. Combined with wool socks and fastening my shoes loosely, they will get me down to *start* temperatures below freezing, provided that it warms up as the day goes on.

You'll want a wind-blocking layer on top, as well, but for those temps, this can be a light wind-blocking jacket with room underneath it for a layer or two of wool (or whatever keeps you warm). When you find a base layer that you really like, you may find yourself wearing it under everything, so the expense is a little easier to justify.

Basically, try it! I think you'll find that the gear you need for sustained sub-freezing riding is largely different from what you'll need to ride in cooler temperatures (also be very very careful of black ice) - so if you ride through December and want to keep going as the real cold weather sets in, well, maybe someone will give you hard-core cold-weather gear for the holidays.