I'm confuse on the brazed on water bottle comment. You don't need a water bottle holder to hold the battery on a Magicshine. Geoman Gear has battery in a pack that is very versatile and can be mount in many spots like the stem, top tube, handlebar, seat post, and etc.
For the flashlight, what are you looking at as in lumens. Most AA battery flashlight will gives only a certain amount of lumens before the battery drain out fast. I say forget the AAA flashlight. There are some AA flashlight that are capable of yielding 200-300 lumens anything over that you may have to look into 18650 battery setup.
All this will come at a cost. What is your price range? and what how bright of a light you think you need?