I like disc brakes for all-weather commuting and these are quite common on hybrids as well a a few cyclo-cross styles.
I prefer the rear brake to be mounted on the horizontal chain-stay tube where it doesn't interfere with the pannier rack. The usual MTB disc position on the seat-stay is harder to integrate with racks.
Your idea of a separate bike for winter is good. You will need a backup bike for when your main bike is out of order.

Budget for all your accessories:
rack, panniers, lock, lights, helmet .
Bike racks come in Front and Rear types. Racks need to be simple, stiff, well triangulated and solidly attached. I like 3 legs, an open framework top for lashing large objects, and a rear lamp bracket.

The brand is not too important. In your price range, all bikes are made from decent Taiwanese aluminium frames manufactures in a handful of big factories with Shimano components. There may be minor differences in components level. Generally Giant, being the biggest manufacturer, get the best deals and highest spec.