I believe a more expensive bike will not significantly out last a cheap bike with the conditions you are describing. A $1,000 fat bike is still vulnerable to the abuses of salt and weather.
As far as comfort goes, the fat bike on asphalt would be a little more comfortable but not enough to justify the price.
I suggest stay with buying used. Try to look for plus sized tires (2.4" or bigger) and disc brakes.
Wash off the bike after bad weather rides and keep parts lubed.
Cheers