You will need different weights depending on what exercise you are doing. For example, a 10kg kettle bell would be perfect for something like bicep curls. However, for a movement like kettle bell swings where you are using much bigger muscle groups, you might want a 30kg kettle bell.
I would get 2 10kg, 2 20kg, 2 30kg and 2 40kg kettle bells. You have a full set of different weights to use then and aren't limited to any exercises you can perform.