Jesuit missionaries from Portugal will appear in Japan in the latter part of the Sengoku Jidai period.
The Portuguese tend to arrive in the southern parts of Japan first, generally they will first appear in Satsuma province around the year 1543. From the island of Kyushu in the southern tip of Japan, they will eventually spread to other districts with ports. This spread is random, but the odds are stacked in favor of ports who are near to the ports where they are already present.
If you own a port where the Portuguese have arrived, then you can meet with the Portuguese. They will teach you the hojutsu advance, which will allow you to recruit Teppou Ashigaru.
Christian Daimyo cannot recruit Yamabushi and Sohei. Towns with a temple may riot, if your Daimyo converts to Christianity, so while you can convert and denounce your faith anytime you like, it is often better not do so.
Christian Daimyo can recruit Teppou Ashigaru at a cheap price of 100 koku instead of the regular price of 200 koku.