Sorry to say but that filter might not be strong enough.. if it says 60 gph it means for fish and turtles are a lot messier then them 
I would suggest Aquaclear 500.. I use it in my 55 gallon and it works great. I do feed my turtles in another tank which might be why its so clean... but I like the design of the filter and its really strong and durable. Some people put 2 aquaclears in their tanks.. (you can reduce the sound by "connecting" plastic or what I did, the top of cd cases, to it)
As for the design, I think you have to experiment... have fun with it! Thats one of my favorite things to do... every water change I consider messing with the setup and giving the turtles more stuff to do in it. I have one of those pre-made "caves" which is a durable plastic like material... problem is, that it floats! But what I did was saw off some of the side and now it fits perfect in the tank. It's designed like exactly to fit.. if nothing is underneath it, it will still stay still if I have a rock on top of it.. and best of all it won't move around! But I still put some rocks underneath just incase.
So I have that for where they bask and some large gravel on the bottom. Right now I'm thinking of what else to add (I used to have fake plants, and other cool "manmade plastic" stuff in it but I think it just cramped up the place). I also wonder whats the best plants to put in.. I would hate if they just ate everything in sight lol. Well I have some log stuff to make it look more real.. they like to sleep with their heads by it. And with the "cave" its nice and mysterious.. when they go in it you really can't see them if your looking straight on. I may have some real plants inside the cave or before the etrance, so when they go in it it'll be like their really hidden 
But yea all i can suggest is look at some pictures from like austinturtlespage and just have fun with it all