Cheap(er) interventions: Bar mitts, they come in drop or flat bar style. Light weight balaclava. Featherweight wind vest. Fenders. Clipless pedals, I prefer eggbeaters with mountain style bike shoes year round. Lots of bright lights. I haven't used studded tires, but if usually roads aren't bad enough in the mid Atlantic to justify them.

Not cheap interventions: disc break/drop bar conversion=new bike. If you have all the money you need, I own the Salsa Vaya and think its great year round. If you are looking at drop bars, disc brakes and fullon mountain tread, the Salsa Fargo might fit that bill. I haven't noticed any decrement in my steering or balance with drop bars while in the hoods.