Great pics. baby turtles are pretty awesome (adults are cool too, they just need a bit more space...)
I figured after I posted that you probably were well aware of any potential problems with humidity/etc., with building previous tanks and such. Now, even more so with the experience. No offense/slight intended, it's just something that I a lot of people looking at big tanks seem to forget/not consider. :/
Anyways, it looks great, and looks like you have plenty of room for the turtles to grow in. (not something many people consider)
If you still want to tinker with the setup, I'd suggest trying to build your own combo waterfall/filter. I think your pics are about the best use I've seen of those tetra viquarium things, but I feel that most DIY builds can accomplish the same results with a better use of space, etc. especially with all the vertical space available in your tank, you could stick something in a corner, use the height to store filter media, and get a pretty impressive waterfall going.
Back to peace lillies, if you already have any (houseplants, etc.), you can often find little juvenile plants underneath/around the main plant. I've gotten starters for paulludaria and such quite a few times using this method. Granted, your setup is big enough you can throw in full-sized plants, so...
But, yeah, great setup, nice pics, and thanks for posting them.