[tenmins]

The other thing to consider is where in Japan you want to live. As with most countries there are many different lifestyles. You can live in the middle of nowhere and be completely immersed in the more traditional style of Japanese living and probably speak no English whatsoever, or you can live in a big city which is pretty much a western lifestyle and most of your Japanese friends will be wanting to practice their English with you.
Or somewhere in-between these two extremes.