hey man i do believe rents are expensive in japan especially if u do want to live in places like tokyo kyoto or osaka.like mmm said u definitely will need something like a 'assurance' money so even more so.I highly reccomend living witha host family.there are tons of ways to get them ur just not lookin hard enuff friend=).But then again 12 months is a long time so i am not sure if ALL the host family will be okay with that,there are host familes that only accept ppl on a long stay.try here
https://www.homestayweb.com/becomeaguest.html.
Hope this helps.Cheers