I would get the biggest Kettle you can afford but a minimum of a 22.5". It is versatile for smoking and grilling and will give you a good idea of what you might want down the road in a dedicated smoker, should you choose to get one.
I have used my 22.5" Kettle One Touch Gold for grilling and smokes of 12+ hours. This forum is a great resource for learning creative ways to utilize the Kettle for various cooking styles. If you can swing a 26.75" Kettle, I would just for the additional cooking space.