The filtration you have sounds fine...plenty of biological filtration and with the HOT filter, it should keep the water clean. But, I do recommend you add on another type of filter, like an Aquaclear 500 or so for added filtration. Other than that, you seem to be set to go.
Oh, and which to get first...personal preference I guess. You could get the discus first and get them settled into the tank, then get the rays later.
-Ryan