Energy is cheap where you live... over here it's 12c in the first tier, and goes up to 24c after that.
Don't know about the plumbing... I tend to keep things as simple as possible, and when you connect that many things, they might start to work against each other. But I am not that knowledgeable with that sort of things.
Your controller setup looks good. Over here, the DIG controller is still sold for $33 at the HD. Keep in mind one limitation of the WC
-- you can only have two things "on" at the same time.
I found that with light and pumps adding some heat to the water I don't need heaters at all... Might be different for a Discus tank though.