A good place to start would be to go out and scrounge or build a housing. You need something that will hold the battery and have a handle if you want to walk around with it. For high power LEDs you will need heat sink(s) so the case should be able to hold (or be a) heatsink(s)
A nice thing about LEDs is that you can build them into anything, it doesn't have to look like a flashlight. It can look like a squid, a UFO, a tool box, a statue of Venus, whatever blows your hair back.
I think of reflectors and lenses as being good to narrow your beam and give it more throw. If you just want broad area lighting, a hemisphere* of light rather than a beam, you can consider skipping them.
I would suggest a couple of switches and a couple of circuits in parallel:
You turn on the lowest setting and it gives you a few little 5 mm LEDs, just enough so you can find the light easy and use it for a nightlight while you are sleeping in a pitch dark place. On that setting the seven amp hour battery might last for a week or two.
The next switch would turn on the medium lights. Lets say about nine one-watt luxeons with no reflectors or lenses, just a diffuser. This would light a room enough to walk around, see colors, you could probably read comfortably next to it.
The third switch would be your high setting. If you need throw, this would be the one with throw, you would give the LEDs reflectors or lenses to make them more directional. three-watt LEDs might work good, or simply more one-watt LEDs. You would use the high setting for field surgery, guiding in helicopters, that kind of thing.
One way to dodge the whole resistors/circuits/math issue is to look for LEDs that are sold for automotive use. They will be ready for 12 volts DC straight out of the box.
I might be able to come up with a picture or two of something similar, I'll see if I can find anything.
