To push 6" of snow you will need a bot that weights at least 150lbs. There are a few videos kicking around the net of a r/c truck with a plow on a driveway pushing 1/8" of snow. And another of a 30lb 1/4 scale truck pushing an inch. To push 6" inches of snow, tracks would be better, or studded tires. And 150 lbs would be the minimum.
You should look at a 2 or 3 foot long rover at least. An 18" long rover with 7" wheels won't climb 6" tall stairs. Trust me. You simply need more length or tracks.
For motors, you will need something like this:
Driving each side of the rover. Use a Vantec, IFI or RS80 to drive them.
Or you could buy 6 of these:
http://www.superdroidrobots.com/shop/it ... 34&catid=7
And run them off a Sabertooth 2X25's.
You'll be getting close to enough power with the small gear motors but you'll want to build heavy.