I think the Yamahas are great machines. Only complaint I've heard is they sometimes can be more finicky starting in cold weather. They use a 20 AH (amp hour) battery because of space issues and isn't enough power in sub zero temps. I personally use a Polaris Sportsman 400, which comes factory with a 30 AH battery and have never had an issue with it starting.
Like stated, 4x4 is necessary and maybe some chains for the wheels?