So when does one start to collect kettles to keep as "lookers"? Is it just the rarity of a certain year or color that I should be looking for?
One collects kettles that one likes. If you want to keep it as a looker, or if you want to use it as a cooker, that is totally up to you. Don't let anyone tell you different.
My opinion is that if it has been cooked on, then it is a cooker.