If you are interested in preserving rare breeds, I suggest that you start with excellent quality breeding stock. There is no "preservation" involved with breeding hatchery quality birds that probably were not purebreds to begin with.
Put "breeding quality" and "rare" together, and you most likely are not going to find any cheap birds that meet both of those requirements.
It would help if you stated what you want the chickens to do: Eggs? Meat? Large? Small? White eggs? Brown eggs? Purple eggs?