The 22.5 inch kettle is going to be your most versatile charcoal Weber. The 26.75 is, supposedly, fantastic, but there are less accessories and less toys (if you're in to that kind of thing). Ditto the Ranch.
On a 22.5 kettle you can grill, roast, smoke, bake, just about anything. It's not as "good" a smoker as a WSM simply because it needs more tending. It's not as good at feeding a crowd as a Ranch. But, it hits a lot of points right in the middle, and it's an incredibly versatile piece of equipment.
The Performer is popular because it jacks the standard 22.5 up a few notches. It has a fantastic work table attached, storage for your utensils, charcoal storage, one touch cleaning, and a gas assist to speed up lighting.
You can smoke on anything. Smoking on a kettle is easy. A smokenator makes it easier, but it's not completely necessary. You'll be able to smoke a butt, cut racks of ribs, or a small brisket no problem. You just won't be able to do an overnight cook without tending the fire.
Stan