[Nyororin]

I don`t see the plan of getting a car inherently flawed on it`s own.
There are just parts of the plan that seem a bit sketchy and hard to pull off.

I`m not really familiar with the laws, but how do things stand with working holiday visas and owning things like cars?

You also have to have a method of disposing of the car when you leave - and if the car has been sold used more than once, it will be VERY hard to find somewhere that will take it off your hands without charging you a fee. When we bought our car - brand new at the dealer - as our old car had been sold used twice it was deemed to be lacking any resale value. The best the dealer could arrange for us was to waive the recycling/disposal fee. We checked at used car dealers and they would have charged us a significant amount to dispose of the car.
Of course, this doesn`t apply when you`re handing the car directly over to another buyer, but you can`t always count on finding one in time... And I would be flat out disgusted if you fell into the seemingly popular path of just abandoning the car when you left Japan, making someone else dish out the money to clean up after you. (Not saying you would do this, but it IS a significant problem - on the same scale as running off without paying the last month`s rent, or running up huge bills just before leaving Japan so you can get out of paying them, etc.)

I also think you have slightly skewed views of just how friendly Japanese roads are toward long distance travel. Sleeping at the side of the road is not really acceptable unless it`s in a designated rest area - which are VERY rare if you`re not on a highway. Driving around for a few months on it`s own isn`t unthinkable - but still, that`s 2 months plus the 4 or 5 months you mention just letting the car stay parked... What about the rest of the year? (as I presume you`ll be in Japan longer than 6 months.)
You can`t just find parking spaces anywhere, even in the countryside - particularly at night if shops with parking lots are closed. And in most of the country, the normal roads are too narrow to stop at the side. Also be warned - using a gas stove (Anything with flame) on the side of the road or in a parking lot is enough to get you arrested. Japan is VERY strict when it comes to anything involving fire. When camping, you aren`t even allowed to cook anything at your spot - it has to be done inside a building or in a specific designated area. If a fire happens and someone happens to have seen you cooking at the side of the road - expect your life to basically be destroyed, regardless of whether you were at fault or not.

Anyway - as for speeding in Japan, the posted speeds are far closer to recommendations than actual limits, it seems. Virtually everyone drives over the speed, and the only time I have actually heard of someone being pulled over was when they were going more than double the posted speed or recklessly driving. It really only strongly comes into play in an accident - if you were going over, then things are definitely not going to go in your favor.
Driving about the same speed as the other drivers is normal, regardless of how far over they are driving. (I regularly drive 120km/h on 60km/h posted highways. Just like everyone else in the right lane. But that`s on the highway.)

I may sound negative, but I definitely think that driving about is a wonderful way to see Japan, especially outside the cities. I love my car, and that is how we travel. I wouldn`t give it up for anything, even for inner city travel. Then again, I don`t live in Tokyo - which brings me to another point. Why end up in Tokyo? You could base yourself out of anywhere in Japan. Jobs are just as plentiful in other big cities - actually, easier to get around here as there isn`t the huge community of foreigners like there is in Tokyo.