Well, I don't know about the USAF, but when I was stationed in Yokosuka with the Navy from 98-01, unmarried E4 and below could not own a car in Japan. Only E5 and above or E4 and below if married and the command approves a request for car ownership.
But, that was 10-13 years ago, so it may be different now.
But, there is no limit to what you can own as long as you can pay for the associated costs with car ownership in Japan. It's not cheap, especially if you own a car like a GT-R.
Tax classes for cars fall under several different categories. Overall car size, engine displacement, weight, etc. Plus, you have to have a place to park before you can buy a car if you live off base.
Not trying to discourage you, just trying to give you a heads up on what to expect should you get orders here.
Now, as for the bases...Yokota is more central Japan so weather is decent. Hot and humid summers and mild winters.
Misawa on the other hand is up on the north end of Honshu island in Japan (the biggest of the 4 main islands of Japan) and they have some pretty harsh winters up there.