The trouble is, according to the fluff, most Buddhist sects see magic as merely another part of reality that must be overcome to find enlightenment. The Buddhist tradition in Street Magic, which is an exception to that, is Vajrayana, or Tantric, Buddhism, which would not really fit a samurai nearly as well. There would be enclaves of it in Japan, just as with Christianity, but if your main concern is an authentic samurai-type character, Shinto would probably fit much better.