Yes you need to build a above tank basking platform. There's slot of missinformation saying not to fill tanks all the way for turtles but that depends on species. RES are dominate swimmer they need as much water space as possible.
If you start feeding her right and giving her right water space and proper basking area she will grow some more. You need to also remember since she is a female it will lay eggs.
For substrate pick whatever you can use the ceramic tile that will help on clean up, but with sand it's not really a mess FYI. All my turtle tanks have sand and there crystal clear.