As for the sand, some people debate that it be supportive of the turtle's digestive system. My personal opinion is that there is no proof and I would rather be safe than sorry to attempt using a substrate of sand. When in doubt, I think of the natural environment of the species I'm taking care of. Most aquatic turtles never deal with sand. Mostly mud.
I would prefer to go without it. Turtles are scavengers so they eat almost anything. It wouldn't be unlikely for your turtle to eat the sand or pebbles of substrate. I would recommend large river stones. Big enough so that your turtle won't run the risk of digesting it. I've seen on these very forums a case where someone had a turtle who was sick and couldn't move their hind legs. It got an ultrasound done and found out there were pebbles gathered and stored in the lower body of the turtle. It couldn't digest it and it made it disabled.
I'm glad you came to ask me or just came on the forums. It's important to do proper research and ask the right questions when it comes to caring for any life.