Midlayer is the cheapest and most versatile way to improve insulation. A long sleeved jersey is ideal, it doesnt need to be a fancy one, the material of midlayers is much less important than that of base or shell layers.
A neck zip is useful but a rear pocket (under the shell) is of little use.
My winter gear involves a merino baselayer, polyester jersey, woollen sweater and wind/waterproof.