I've been waiting to see if someone more qualified to talk about Sengoku-era religious commentary would show up, but since no one has, I guess I will give you the answer that I can give you. I hope this is still in time to be somewhat useful for your campaign.

The guy you should be looking into is named Francis Xavier; he was a Jesuit missionary to Japan (among other places) in the late 16th century, and as he was fairly prominent his letters are more well preserved than a lot of his contemporaries. For example, here is one from the archives of Fordham University.

You can see that the letter may be more focused on conversion than culture shock than you might want it to be, but it is a place to start. For example:

Japan is a very large empire entirely composed of islands. One language is spoken throughout, not very difficult to learn. This country was discovered by the Portuguese eight or nine years ago. The Japanese are very ambitious of honors and distinctions, and think themselves superior to all nations in military glory and valor. They prize and honor all that has to do with war, and all such things, and there is nothing of which they are so proud as of weapons adorned with gold and silver. They always wear swords and daggers both in and out of the house, and when they go to sleep they hang them at the bed's head. In short, they value arms more than any people I have ever seen. They are excellent archers, and usually fight on foot, though there is no lack of horses in the country. They are very polite to each other, but not to foreigners, whom they utterly despise. They spend their means on arms, bodily adornment, and on a number of attendants, and do not in the least care to save money. They are, in short, a very warlike people, and engaged in continual wars among themselves; the most powerful in arms bearing the most extensive sway. They have all one sovereign, although for one hundred and fifty years past the princes have ceased to obey him, and this is the cause of their perpetual feuds.

Besides Xavier and his fellow missionaries, the literal first Europeans to set foot on Japan that I am aware of are Portuguese sailors who wrecked with a Chinese junk on the island of Tanegashima, just off Kagoshima in Kyushu, named António Mota and Francisco Zeimoto. The were incidentally carrying a firearm, and according to a Japanese record called the Teppouki, demonstrated it to the local lord who was impressed and bought it off of them. He instructed his local blacksmith to learn to make copies of it, and after some drama (one history says that the man gave his daughter to another Portuguese merchant for the final secrets of their manufacture) he began producing copies that soon disseminated across Japan.

Of course, the actual evidence is much less clear on whether Tanegashima was really that important to the spread of firearms in Japan, as the Teppouki was written as a celebration of the Tanegashima clan, but it still could be interesting to you. I can try to dig up a translation for you if so.