There is more to flashlights than brightness. Color (color temp), ability to render the colors of objects (color rendering index [CRI]), and intensity (Candela) are some of the most common measurements.
There are also things like how tight you want your beam and the type of optic you want. Do you want a reflector or TRI optic? There are so many little things that go into the perfect flashlight for each individual.
There is! Check out intl-outdoor.com for a very wide variety of flashlight bodies and a large range of emitters. I have never DIYed my own flashlights however I plan on changing that in thr future.