It's an extended story of the danger of the messianic urge told primarily from the perspective of a messiah ("the myth of the messiah," as Herbert put it) more or less. There's more to it than that, of course, but that will do for a start. I don't think it's a direct allegory for anything Islamic or Arabic except to the extent that there's obviously a lot of influence from both in the cultural setting, language, and so forth.
As for the other books: the first is by far the best, because it had a strong editor during its serial publication in an SF magazine, and Herbert really needed editing. Most people would be happy finishing with that one. But if you stop there you're not getting the full story he wanted to tell (except if you read the first very, very carefully; he does telegraph the rst of the story, but -- and here the editor, who had his own very strong ideas of how SF should be written, probably failed him -- the narrative of the story seems to lead to almost the opposite conclusion). So I think you need to read the second book (Dune Messiah) to finish off the story told in Dune.
If you didn't hate the second, and some do, I'd then read the third one (Children of Dune), and at that point seriously assess how much your time is worth in terms of continuing. The fourth (God Emperor of Dune) has some interesting concepts and a few moments of good writing, but there's an awful lot of ridiculousness to slog through to get to those moments, and I'd say it's a coin-flip. After that (Heretics of Dune, Chapterhouse Dune) they're impenetrable and pointless, and you shouldn't bother.
As for the son's books...a world of no.