I once considered building a 3x1 but the fact that you still need a rear derailuer to take up chain slack steered me away. Since you talk about changing conditions and varied terrain you might want to keep all the gears for a while, just to see what you really need. There is so much difference between lightly loaded nice days on flat sections and being all bundled up and heavily loaded while climibing hills in a snow storm.

In reality I think you can do as much or as little to winterize a bike as you want. I've had some (but actually very little) trouble with open gears or cable freezing or anything like that. I rode for a while with just an old MTB tire on the front and a semi agressive 1.5 or 1.75 on the rear. I also park outside at work so I've had to clean snow off the bike and ride home in heavy snow with big hills. In fact this will be my first year with discs and studded tires, though I do admit that rim brakes suck in any kind of standing snow.

So I think that before you start building a bike you should put the thing together with the parts you have, find some tires you think might work and give it a try. See what you need before you change all that much. Maybe you make a few ajustments, maybe you want quality studded tires, hydraulic disc brakes, and an internally geared hub. Or maybe you decide that it is a pain in the rear and unpractical.

Also don't forget to think about clothing. I'm sure there is all sort of advice on here about layered synthetic materials, about wicking and soft shells and shoes and so on.