I know stinkpot musk turtles stay very small, but I really don't think you should be getting a turtle yet. You need to google around for a turtle forum, and for caresheets that apply to the specific species of turtle you get. If you don't do your research, you'll end up with a very smelly, very unhappy, deformed turtle in a dirty, ugly tank. Invest a few hours of reading, write down everything you'd need, and don't cut corners, and you can give your grandmother a great little pet that'll last the rest of her life, and maybe yours, too.
The kind of info you should be looking for is:
-What temperature should the water be?
-How deep should the water be?
-Should there be substrate, and what?
-Should there be plants or hiding nooks under the water, and what would be safe, with no risk of drowning your turtle?
-How much time does your turtle spend in the water and on land?
-What does your turtle need to eat to be in pristine health? It will be a variety of foods, for sure, deffinately not just pellets-in-a-can.
-How dirty is this turtle, and what kind of filtration and water changing schedule will it need?
-How/where will you find healthy turtles to buy?
-What kind of lighting does the turtle need, UVB, etc.
-How big will your turtle get, and how much room will it need as an adult?
Chime in if I've missed anything major. Turtles aren't beginner pets; They're messy, they need lots of cleaning, and you can't ever walk away from them; They can live at least as long as a person, and deformities and neglectful conditions can affect them the rest of their lives. Owning a turtle is a commitment, a very long commitment, unless you can find a good home for it.
Also, if you could manage it, I find turtles look much cuter with other turtles. Just throwing it out there
-Bryce