Legend of the Five Rings
I'm no expert on L5R, but I did sleep at a Holiday Inn Express last night...
DISCLAIMER: What follows is discussion in an earnest attempt to answer the questions posed by Varsovian in the original post, and should not be construed as advertising or endorsement of the product in any way.
From what I can see, the Fourth Edition is the latest, and, yes, it follows pretty much the path you've described - expensive, and with supplements which, if not essential, are at least highly desirable. Apparently, (and I just learned this) FFG may be the publisher, but the trademarks associated with the game are still held by Alderac Entertainment Group.
The newest core book weighs in at over 300 pages, and contains GM and player materials. My quick peek at it incited feelings of "new game lust", but I went and took a cold shower and should be okay in a minute or two.
There is a "beginner box", but, in my own opinion, it asks for too much of a financial commitment from the "average" beginner.
From what I can see of the game, it looks sumptuously done, and, depending on one's preference and desires, may well be worth the cost, I don't know. That would have to be an individual decision.
If I had to compare it to anything else I've seen for richness of detail and setting, it would be 7th Sea, and small wonder, since John Wick was at one point involved with both projects. Though the credits for 4th Edition do not bear his name anywhere, his earlier involvement with the project seems all too clear, at least to me.
Most games about ancient Japan take the route of making the setting about some fantastical version of that proud and unique nation. This can be taken for "poor research" or "stereotypes", but I prefer it to mean the games' designers wanted more latitude to portray a setting that diverged in significant ways from Japan's actual history.
Bushido and Ronin, two games which are in similar milieu, may serve as a basis for comparison. All three games use a fictionalized Japanese setting which tends more toward the cinematic than the accurate. That's not necessarily a bad thing: I grew up watching Japanese television, and really get the feeling the designers of L5R were trying to create a more "action/adventure" version of ancient Japan that may have been heavily influenced by some of what the designers might have seen in film and video.