Your tank build sounds nice, I'm excited for you
. Here is my tank I had custom made. The dimensions are 36x36x20.
I would highly recommend not placing your overflow box inside the tank. The reason is not only does it take away from the space inside but, it creates shadows which in turn eliminate "real estate" space for your coral to gro later on. If I had to design mine all over the only thing I would change is having the overflow box on the outside.
You will love the shallow tank, it is soooo much easier to clean and maintain your tank! I won't ever get a deep tank. Also, IMO rimless looks very nice and being able to look down on your coral is amazing in itself.
Also, I would recommend against using only one Luminarc light (like someone else suggested), I tried using only one Luminarc in my tank and the coverage was awful, the center was well lit while the edges of the tank and upper reef were very shaded. I am now using two 250w lamps one with a 20k helios and the other with a 14k pheonix, I love it!