Welcome! It's great that you're doing your homework before hand!
RES do much better on their own, so you'll be best just getting one.
Sounds like you've got a pretty good setup (and perfect for one RES). Sounds like you've got pretty much everything covered.
For the bottom of the tank, bare bottom is the easiest to clean.
I use about 20 large river rocks in my tanks. Give the turtle something to push around, but still pretty easy to clean.
Sand look really nice. I've got sand in my fish tank, and it's easy to syphon debris off the bottom. Only thing to keep in mind is that the turtle can kick up the sand, so you'll want to make sure the filter intake is a little ways above the bottom so you don't get sand in the filter.
The only other thing with sand is that some sand (particularly play sand) can take a while to clean initially. But the good news is you only have to do that once
