you wont need to worry about timing too much since both light and electricity both go fast enough
if you are making a "trip line" any laser will do pretty much, unless you go up to like a half mile, you may want to use a more powerful one (5mW)
instead of using a photo resistor, id recommend a photo transistor, you can then get one to filter colors except for the lasers color, resistors vary values, while a transistor is more like a switch
Light is measured in Lumens and Lux, you can read up but you wont need it much for this.
Hard part for you is going to hook the sensors up to the camera button, a relay may easiest for you, unless you know how to use transistors,