The box stores sell white Tyvek front-zip coveralls for $8-10 that are wind and waterproof; work great, wear all the warm clothes you already own underneath. You can sit all day without a wet rear end, and a pair or two will last a couple of years.
If not in love with that option, I would opt for a white snowmobile suit...good luck on finding one of those.
All ya' need after that is a white stocking cap/balaclava and you are good to go.
