I would go 2 blues for every white.
Cree 3W LED's are the most popular.
http://ledsupply.com/creexpg-w139.php They come on an aluminium disk, its about 3/4" diameter.
Don't make a single strip, make an array.
I would buy dimmable drivers, its just so much easier and they are cheap all things considered.
Optics... try some of all and figure out what works best, makes it easier to give a smaller spot more light if you have a demanding coral.
No reflectors, they focus down on there own.
use a heatsink, fans depend on how many LEDs you have and the size/efficiency of your heatsink.
you can use an epoxy or you can screw the LEDs down. I would drill and tap holes and use machine screws to hole them down.
18g is plenty fine for wire.
I am building a small array of just whites and using T5 for the blues, actinic and fiji purple kind of tubes. Waiting on the XPG to come in blue.