I buy almost all my new gear in Tokyo at either Yodobashi or Bic Camera.
Picked up a D7100 a few months ago, and the firmware can only be set to English or Japanese. If you want to verify, play with the display model and make sure you can change it to English. Charger is fine and works in Singapore. The only issue you might have is that the manual comes in Japanese. Just download the English version from Nikon's site.
Depending on which salesperson you get, they may or may not tell you about their rewards program. If you sign up, you get 10% of the purchase price put on the card to be used as a store credit for your next purchase. If you have no intention of buying anything else, take your passport and plane tickets. They will usually knock 10% off the price if you tell them you are visiting and don't want to purchase any other items.