just another data point:
I mow and plow with a JD425 AWS garden tractor - gas, 2 WD with locking diff. For snow plowing, she is equipped with a 54" blade, hydraulic angle and lift, a couple of little suitcase weights and tire chains. I've plowed snow deeper than the blade is tall with no problems. The all-wheel-steer makes her extremely agile and she will plow circles around much bigger machines in confined spaces. Being a good neighbor, I plow all my neighbors drives plus mine (7 drives about 500 feet total) in about an hour, and this is being very careful around their garages and cars. 450 feet of open drive would take considerably less time to do.
Also, for mowing around trees, gardens, bushes, etc. the AWS can't be beat. I mow 2 acres in 1.5 hours with ease. I have a lot of trees and gardnes, but only have to trim with a weedeater about once a month. Coupled with hydrostatic drive, the AWS takes care of most of that.
So, I'd say you don't need 4WD. But, do get a heavy machine. Weight to the ground is paramount for plowing. 20 will be plenty. AWS (or similar systems) should also be given serious thought too. If you even think you'd like to have a loader or 3pt implements, make sure you get a machine that will support that. Don't buy too small and regret it. But, don't buy more than you'll ever use either.