In answer to your questions:
1. Yes you could use "0.00" or "free" if you wish to indicate no cost for the item.
2. Yes you can edit the field values in the ClassiPress admin dashboard and remove all countries other than Japan, and also replace all of the default US "States" with Japan related regions (as well as change the title itself)
3. I don't believe we have any current Japanese based sites listed on our showcase, however there are some from various countries
here. Thanks.