If it were my bike I wouldn't upgrade the drivetrain (cassette, rims) until summer comes. Might as well ride the rest of the (gritty) winter/spring on the existing drive-train while you consider what you will do with it.

My Univega is very happy to let me wear out its current set of 27-inch wheels and old freewheel...I bought it last summer and given the way it's working as a winter commuter it may even make it through *next* winter before I have to consider a drive-train upgrade/refresh.

I would, however, change/replace the brake pads, cables, housing, brake hoods, handlebar tape and possibly tubes, tires and rim tape, depending on condition.