How are you going to deal with accountants and lawyers without decent Japanese skills?
Average monthly living expense depends on your lifestyle and choice of location, of course. Figure subsistence wage around 180,000 yen/month as a rough estimate.
Rent 80,000 (one person, NOT near center of the city)
Utilities 20,000
Phone (with Internet) 5000
Food 30,000
Insurance ?
Local transportation 20,000
Other things depend on you