This clearly means the number of colors you use is not as important as how you use them. On the other hand, it is sometimes important to get quick easy access to the whole rainbow or the most used colors, for instance if you paint on location. So you may add some additional colors, depending on the subject. Get any complete water colorist set and you are done. Just please choose professional fine art colors .
Every brush is welcome for nice watercolors.
But this said, it's convenient to have some good brushes to wet the paper or detail the model. Brushes are in fact more important for watercolors than for oil painting. A good watercolor brush can take a good load of water and enables precise outlines. I have some synthetic brushes to take outside. But the nec plus ultra are Kolinsky marten.