Kevin,
Wrote a nice long reply to this on Christmas eve but closed the window instead of hitting the POST button first. Didn't have time to post again until now.
Let's see if I can recall everything (and make it a lot shorter than my original post)... EDIT. It isn't too much shorter.
First, it would be beneficial to know where you live. How much snow, how much acreage, terrain (hills, flat), wooded or open. Will the ATV be used almost always for doing chores or do you want do do a bit of "recreational" riding?
Is there a Mrs. Kevin that will be driving this or anyone that would prefer an auto vs. manual shifting?
Hate to ask so many questions but the answers would help us better define a machine for you. Initially, I would think a 250cc would be minimum but would work, a 300-500 would take care of anything you've listed.
For the plow, it'll depend on the ATV and the amount of snow but generally, a 52" works nicely. Snowblowers are veerrryy expensive for an ATV and generally, aren't worth the price. Warn or Moose brands would be my first 2 choices, followed by a Cycle Country. For a gravel driveway, you should be OK if the driveway is relatively flat. All the plows come with skid shoes that you can raise to keep the blade off the stone.
For lawn chores, a small cart or wagon (10-20 cu/ft) is handy for carrying tools and such. A small trailer (4x8, 5x6 or somewhere thereabouts) is handy for hauling dirt, mulch, trees, bushes and other bulky dirty stuff.