When you are at the store and are buying the camera, what is the most important thing that you should do apart from paying the money?
The answer is to inspect your camera thoroughly. I am not necessarily saying that the manufacturer will cheat on you, but there is a chance that there are manufacturing errors. Since the camera is so expensive, you should make sure what you bought is what you want.
It is also a good idea to try taking a few shoots in the store so that you can 100% understand how the camera works—try different functions!