When picking a camera to learn on one thing to keep in mind is that you’ll be learning on it
The gear is only as good as your knowledge in how to use it
So picking something with room to grow and potentially in the future grow out of without getting something that’s super expensive (to you) is important
Go right ahead and pick up the 6400 and begin your journey into interchangeable lens cameras
Just be warned: it’s really easy to come down with a case of GAS (Gear Acquisition Syndrome) when you jump into the world of interchangeable lens cameras. Try to avoid that while learning if you can