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. These quality brushes retain water and color in a unique manner. Consider brushes as an investment.