First of all: you are going to LOVE Tokyo. It is awesome! Second: I was there a month ago for 5 days or something and - not to scare you - blew way more money than 72.000 yen. But then again, I stayed in a normal hotel which was 58.000 for 5 days and I spent all my money on karaoke and unlimited beer during karaoke (which sets you back 40 euros or something, but I cant remember because I was sort of drunk all the time ).
Beer from your friendly hotel/hostel vending machine will cost you 220 yen. Beer in a bar/restaurant around 500 yen.
A cheap bowl of ramen or conveyor belt sushi will set you back around 1000 yen (and green tea/water is free most of the time).
worn panties cost around 500 yen
I am afraid that I cannot answer your phone question. I just used my european sim and tried to use wifi as much as I could.
Plus, as said before by others, I am sure that the square sounds crew can help you with all your questions!
Enjoy your trip and gig!