I like your concept Nuccio. According to the article on this link there were four primary classes in medieval Japan during the time of the game - Samurai, Farmer/Peasant, Artisan and Merchant.
http://asianhistory.about.com/od/jap...JapanClass.htm
To answer your question about Merchants, they were mostly considered parasites until the Samurai started loosing their influence.
This article mentions the rice economy but also notes the use of copper cash imported from China at the end of the thirteenth century, so there must of have been something spent at the local tavern besides bags of Rice.
http://aboutjapan.japansociety.org/c...omachi_periods
So while Naisho and Emishi are right about rice, land and goods being the primary exchange medium, I don't think it would strain the history of the era too much to have a few coins floating around that could be used to pay a tax.
