Gentlemen
Just somewhat intrigued.
Did the agencies discuss Alien Registration with you??
It used to be (maybe still is) that you cannot rent an apartment in Japan ie take a lease unless you have Alien Registration, nor can you pay Japanese tax (which at 15-20% is far less than many countries.)
Of course if you reckon you can avoid paying any tax anywhere in the world that's fine - just hope you've done your homework!
IF you can rent then be prepared for higher initial costs than in the west (bond, key money, "thank you money", maybe others.)
Rent wise everything is based on square metres: per month look at (USD):
20m, 5 min walk from Shinagawa, fully furnished single room 2200;
50m, 15 min walk from Shinagawa, part furnished 1500;
70m, 10 min train from Shinagawa, unfurnished 1500.
Just a rough starting point.