Thanks for the feedback. I did some more reading last night and am now leaning towards filling the majority of my tank with land. I am looking for a exo terrarium - it will make cleaning the tank easier if I can access it from the front as opposed to the top.
I'll likely take the easy way on this one and pick up a pre-made waterfall/filter. I've seen some nice ones for turtles, but we will see - I do have a general plan in mind if I attempt to build; the major issue is timing on this one, I don't have the luxury of time in playing as the person who I'm getting them from is wanting to be rid of them, but wants to keep the tank that they are in.
I've learned that they are poor swimmers so I won't be needing a "swimming hole" I do however want a way to filter their wadding pool in an attempt to make looking after them easier and less time consuming.