I'm swinging back and forth between individual heat sinks, one per LED and one big heat sink, serving all of the LED's. Ideally, it seems that a big heat sink is the way to go, but they aren't cheap. That is what made me wonder about using small heatsinks, each with its own cooling fan. Economically that looks good, but I have no idea what the power limitations are for them.
Evil, I hope to keep the fixture about 24 inches from the substrate, and use lenses to concentrate the LED output into the appropriate area. Does a LED with a lens produce anything close to equal intensity throughout the area lighted by the beam?