Winter biking is fairly hard on a bike because the salt on the roads gets all over your frame and drivetrain. Therefore I prefer a dedicated beater bike for winter with studded tires. An internal gear hub or fixie is also not a bad option to consider for winter. (Another option is a different wheel set for summer and winter that you can quickly swap depending on conditions.)

$700 isn't going to go far if you are buying new, due to all the add-on bits that you need for commuting. The best option would probably be to get a vintage mountain bike with no suspension and purchase two new sets of tires, a Hakka W106 studded tire set for winter and a smooth puncture-proof tire set for summer. Attach fenders and some sort of rack, get a good flashlight and twofish flashlight holder, a Superflash blinky, and you are good to go. If you are handy, you can make your own set of panniers. I made a pair out of ginger buckets. It gets dark here by 4:30 in winter, so good lights are imperative for winter commuting.

The tires are an often under-looked piece of the equation--don't scrimp there. You can start off with the conventional mountain bike drivetrain, but consider upgrading to an internal gear hub if you plan to use the bike in winter.

I'm guessing on prices but here's what I'd budget if starting from scratch:

Craigslist Mountain bike $120
Hakka studded tires $160
Summer tires $100
Rack $50
Fenders $40
Superflash $25
Twofish $15

I guaranty that with awesome smooth summer tires plus awesome studded tires, an old mountain bike would blow away any 2010 models you are looking at in your price range.