About Saruman and the Shire

Firstly, Saruman, I believe, had fallen into darkness (though not openly), far before Gandalf discovered that he had. FAR before it. The Shire, defended by the Rangers (and, I may add, a VERY select cadre of them), was otherwise defenceless. Now, when Gandalf mentioned that he had found the Ring in the possession of a Hobbit (Though Saruman had been spying before then), it will have largely increased Saruman's interest, not in the Shire, but in it's inhabitants. Ergo, it is no small wonder that he found them to be hardy folk. Subsequently, as had been mentioned in Hobbiton by Gandalf to Frodo, Saruman would have realised the prospect of forcing the Hobbits into slave labour. And, surprise surprise, that is what he did (more or less), after being ousted from power by Gandalf and the Fellowship (with the invaluable help of the Rohirrim). So there you go, a rather plausible reason for Saruman's interest in the Shire, cheap slave labour.