Went there last year in March. Great time to go and you miss Golden Week. I could give some suggestions, but it really depends on what you want to do & see there. Japan is mostly cash and not credit card, but 7-11 & Lawsons have atms that you can get currency from for a better exchange than the airport. You can probably get some money ahead of time from your bank here and get a good exchange rate; you usually have to order it but its an easy way to have cash off the bat until you need more. Definitely get a Suica or Pasmo card, makes getting around much faster and easier plus you can get back what you don't spend at the airport on the way back.
Any particular areas you are going to be staying?