Lets see, 300 million Yen is roughly 3 million dollars. I guess you could live pretty comfortably on that. However if you're not careful you could blow through that pretty quick and not much to show for it.
Maybe. It doesn't go nearly as far as you think. It MIGHT be enough to maintain your standard of living for the rest of your life without working if you don't go too crazy.
Kind of depends what era you've living in. Even 1% interest would be $30,000 year, might be comparable to a shitty office job. OTOH getting a safe 1% above inflation can be challenging these days. You can get higher returns but with higher risk.
OTOH again, if you budget strictly, you could just spend $30,000 a year, and it would last 100 years, and you're not living that long. Any interest is a bonus on top of that.