Sounds like a great trip plan, very jealous! One thing is though from Hirafu you won't be able to ski Western Hokkaido without being away for at least a few days, places like Furano, Sahoro, Tomamu, Kamui, Asahi Dake, etc.
On the car hire front here is my details. Booked through these guys:
KAKUYASU RENT-A-CAR HOKKAIDO who are an agent but with better prices than direct, the car hire was actually through ORIX. Incredibly helpful on the email front. Original booking was when we only had 3 people so went for a small 4 door car with 4WD including ski racks, english GPS, snow tyres and included insurance (nothing more required) for 13 days for 59,600 Yen (approx $50 a day). Kind of car would have been a Honda Fit (Jazz) 4WD and would have been fine with 3 people with our ski's on the roof and the rest of the gear inside.
Anyway, ended up having 2 more people join, so we now had 5 all with gear. The crazy thing is they only offer the ski racks on the smaller cars, so we couldn't even get away with a station wagon (Toyota Wish). So we ended up with a 7/8 seater van with all our gear inside, Nissan Serena it was or you could get the Stepwagon. Cost for the 13 days was 114,300 Yen (approx $92 a day). It was perfect. Fit all our gear which was a lot. It was 390 Yen (or something like that) for the ETC card for the whole rental (NOT per day!) which was great as then didn't need cash and could just go through the ETC lanes on the freeway. We then paid the bill when we returned the car which was about $70 for our express way use. They have an unlimited Hokkaido express way pass but that isn't really good value.
Those prices were all inclusive with a low excess, no need for any other insurance options. Snow tyres were great, no need for chains. Oh and all the above was pick up/drop off New Chitose Airport.
Any questions on driving in Hokkaido in the winter (or anything else skiing Hokkaido) just let me know.