is this an update and revision of the old RPG Bushido? It sorta looked it.
No, it's not. Sengoku is based on the Fuzion system. You can check the core mechanics by downloading it off the web, as it's a free system. Just search under Fuzion and you should find it. I myself dislike the fuzion and won't touch it with a 10' pole, but it does have its adherents. However, it's a system that desperately needs updating; when it came out, it was a good system, one of the better ones in its day, but now it's hopelesslessly outdated. At least, that's the impression I've gotten from reading Sengoku.
As far as Sengoku goes, it's a good reference book on the sengoku period of Japan, being a comprehensive guide to social, economic, political, and military histoy. Still, at the price it's being charged, I'd think it would be a better investment to either get a public library card and rent the Cambridge History of Japan's volume on the period, or buy Tumbull's book on the same period.
That is, if you are interested in the period in more than game mechanical way. If you are looking for a game on the period, try the free fuzion rules and see if you like it. If you do like it, it's economical to buy Sengoku; otherwise, don't bother and just buy a real history book.
P.S. What is "FLGS?" Is that a gaming store?