I would think long and hard about planning to come and stay forever. That was my plan. I'm now 2 years in and I'm staying because I have nothing to go back to. My university degree was to get me to Japan.
I think the biggest factor is where your placement is in Japan. I'm in a rural area in what is considered a rural prefecture. I was not prepared for the reality for where I was placed. In school and through media you learn about the big cities, Tokyo, Osaka, ect. But the culture and way of life is different in the rural areas.
I have to stop here and state, that I love where I live and I enjoy 90% of my job. But I know that I cant live here forever. It might have been different if I had been in a major city (I have a friend in Osaka, who has faced none of the problems I have in the rural area).
I suggest that you carefully weigh your university degree. Get something that will serve you better in the future should you decide that you dont want to/cant stay any longer.