If I was to build up a bike for serious long distance riding, I'd start with Surly's Long Haul Trucker frame and build up from there. The LHT comes as a complete bike too with the Slickasaurus tires mentioned previously and lots of attachment points.

You can special order through REI if my contact there is correct.

For snow, forget that other tire, check out the Endomorph. I'm in SOCAL, what do I know about riding in snow