Koi need at minimum 36" depth, and 48" is better. Of course, many of ours (including mine) are doing just fine with shallower ponds. My pond is 30" with one 3'x3' section at 36". My goal was the whole thing at 36" but we hit clay at 30", and the guys just couldn't keep chopping through that - esp since I was paying them by the hour, plus my fish were stressed in tiny holding tanks until the new pond got dug.
Part of the reason is that koi develop their muscles by swimming up and down, not side to side, and they can only do that in a deeper pond. Of course, if you are not showing fish, and don't care a diddle about muscular fish, who cares? Another reason for the depth is to keep water temperature fairly stable. That, in my mind, is more important than developing Mr. Atlas type fish.
You could devote one end of the pond to have shelves, and let that be your lily / plant area, and the rest with straight down sides. Of course, that makes the liner folding all the more intricate.
My pond's sides go straight down, with one end a shelf at 24". The lilies and other objects in the middle of the pond sit on milk crates, which are quite sturdy, and which I have cut some of the "X" things out of, to make enough room for the koi to swim through, and provide hiding places.
