1. You can train No-Dachi after you get the Legendary Swordsman event. Basically, one of your warriors in one of your units will gain enough experience to become a Legendary swordsman. Use your Daimyo unit to kill as many enemy units as possible and you should get it fairly quickly. You'll like the video pop-up that goes with the event.

2. To convert to Christianity you have to be visited by the Portugese (~1543) or the Dutch (but the Dutch show up late in the game). Converting to Christianity will give you a lot of unrest until you build churches and convert the masses. You wont be able to build more Buddhist temples* but you will get access to gunpowder units. Skip the arquebusiers and go straight for musketeers (requires a Cathedral).

enjoy!!

*You actually can build more Buddhist temples but that will be counted as a conversion back to Buddhism.