There are some areas that you really need a car to see but since it sounded like you haven't been to Japan before I'd say skip it, there is so much to see using public transportation that it is not worth the hassle. I think you could navigate no problem if you have a little patience. I used to estimate 1000 yen an hour for expressways plus the same for gas so it really adds up unless you have 3-4 people and parking in even small cities is crazily expensive. Stick to other means of transport for now.