We drove in Hokkaido once. It is nice! There is GPS so it is easy to go to most place. However, I would suggest you to find all the telephone number of the places you want to go first.
The toll of the highway is quite high.
in the city, you needn't drive. the train is good and convenience enough.
Not sure if you are still interested, but if you are really considering 自駕遊 you should visit this website. There is a lot of trip planning information.
We just visit Hokkaido in Nov by 自駕遊. I don't think it was particularly difficult (as long as you rent a car with English GPS). We copied phone number and also mapcode before we went, so no problem navigating.
Driving on snow is not terribly difficult as long as you drive slow. If you have prior winter driving experience in US/Canada then even better. You can rent a car with snow tires / snow chains / 4-wheel drive, which helps a lot in the snow.