My general answer is that if you can hike with a loaded pack for 4 - 5 hours (without feeling exhausted) and are altitude acclimatized, then you are probably ready. My specific answer is that you never really know that you're ready until you are standing on the summit, and then get safely back down. While that may sound facetious, it really is true. Conditions change day to day, you feel better/stronger on some dayts than others, gravity fluctuates, etc. I've gone on long multi-peak climbs where people summit 2-3 peaks, then can't make the next one. Also, commitment to make the summit is almost as important as conditioning - you probably won't feel great and pushing through it (though not beyond your limits) usually determines whether or not you go to the top.
Water: I bring 2 liter bottles and fill as I go. A pump is added safety, but you can do a search on this board and decide if it is really necessary.
Headlamps: I like Black Diamond (Moonlight and Zenix are my favorites), but Princeton and Petzel both make several good ones as well. If you go to REI.com, you can compare them all side by side.
