What does Hamlet mean by "readiness is all" (5.2.218)? What does such a statement reflect about him? Could he have made such a statement in Act 1?
Well, first let's look at the entire quotation to put it in context. Hamlet says the following in Act 5, Scene 2:
Not a whit, we defy augury; there's a special providence in the fall of a sparrow. If it be now, tis not to come, if it be not to come, it will be now; if it be not now, yet it will come. The readiness is all. Since no man has aught of what he leaves, what is't to leave betimes? Let be.
In other words, even though the fencing match might be a bad idea or a bad "sign," Hamlet admits here that there is supernatural, even divine, intervention when even the smallest sparrow bird dies. Using MANY words, Hamlet describes how and when this will happen. The point is this: even if it doesn't happen now, or even soon, that sparrow WILL die. MAN WILL DIE, TOO. HAMLET WILL DIE! (It won't be long before he says, "I die, Horatio!" And then he DOES!)
Now, to the importance of your quotation: Hamlet says that the point is not WHEN a man will die, but that he is READY to die. THIS is where your quote comes in: "The readiness is all." If a man is ready to die, there is no reason to put it off because "no man has aught of what he leaves" anyway. So even if that man has riches beyond heaven, it doesn't matter. Hamlet's point is that HE is ready to die while Claudius is not.
Why is this important? Hamlet finally realizes here that supernatural forces, divine forces, and/or fate determine the outcome of a man's life. This is the ONLY time Hamlet is willing to actually ACT, ... and he DOES. In regards to your final question, could he have done this in Act I. ABSOLUTELY NOT! Why? Because Hamlet's tragic flaw is INACTION. Unfortunately, he learns to act too late!
This shows Hamlet's realization that fate is in control. What is going to happen , will happen. Hamlet was not ready to say it in Act I, because his inaction to acting on his revenge was in full force. He was uncertain of what he should do until the end of the play.