And yes when I purchase I want to make sure to buy it keeping in mind that I might want to keep highest light requirement corals
But do you want to purchase your LEDs such that you can grow the highest light of corals... ANYWHERE IN YOUR TANK. Most people do not put corals close to the front glass, not only does it make lighting harder, but also it's very impractical for cleaning the glass there.
I suggested buying one why? Because if you find it's not enough... you can buy ANOTHER, and moving one over so that 2 fit front to back over your tank won't be a huge chore. The price on LEDs isn't going to go through the roof, so you're not saving anything by forcing a buy right now of anything you might want. In fact as you start you're more likely than not going to have your corals on the rock work which hopefully you won't cram against the front, and in time if you find you need more light, you buy another, and bam. Problem solved.
I understand your 31" width of the tank, I know a little something about wide tanks and how to light them. I simply offered one possible solution to your query and gave you the justification for what I said. If you want to buy 2 or 3 or 4 now by all means go for it, I simply wouldn't do it because of the mounting options available the position of your lights isn't set in stone so you can add more later if you feel it's necessary.