The main two Weber kettles are the 47cm and 57cm. They do go bigger (67cm) but these are fairly unusual and also smaller as in the Smokey Joes. The most versatile one is the 57cm and is the one that you will see most people on here talking about. I have a couple of 57cm Webers and a 67cm - but it is the 57cm ones that I use the most.
Of the 57cm models there are two main types, the "Original Kettle" and the "Original Kettle Premium". The price of the Standard 57cm is around £135 online and the Premium is around £165 and although they look different from the outside the actual workings of both are pretty much identical. The bit on the bottom of the Premium is really just an enclosed ash collector where as on the standard model the ash falls through onto the tray that is mounted between the legs below.
To be honest when it comes to temperature control and smoking you will not notice any real difference. The main advantage of the Premium model (for me anyway) is that the cooking grate that is supplied with it has hinged edges that can be lifted up to add more charcoal/wood without having to lift the whole cooking grate off. This is something i use pretty much with every long smoke.
You could buy the cheaper Original at £135 and then buy the hinged grate separately at around £30, but if I were you I would probably just go for the less hassle option from the outset and buy the Premium at £165.
I hope this helps