Hi again, glad to see you back and talking through your ideas. First, a hen that's been laying for a year or two would traditionally be called a "stewing hen" and for some recipes is the only chicken that is considered suitable such as
Coq au Vin. It is my mother's favorite bird for making chicken and noodles because it's age gives it much more flavor. She would cook it in a pressure cooker, but a slow cooker would achieve the same thing. If you try to buy a stewing chicken now days you will pay a big premium for it. Another use would be for stock. If you're expecting it to be anything like a fryer, you'll be very disappointed. I think that the term dual purpose is meant to convey that the bird can be raised for either a meat bird or a layer. If you choose to raise it as a meat bird you would feed it accordingly and butcher it at a young age so it's tender. (Others... please correct me if I'm wrong about this.) Our grandparents and great grandparents were far less picky, I believe, and would never waste the meat. Thus they butchered and ate the older hens when they no longer layed well. My grandmother, born 1875, certainly did, and all 9 of her children followed suit during the depression.
The flock rotation makes sense to me, and there are several breeds that will do well for you. Your plan will certainly stagger the effects of age, molting, etc. If you're less interested in eating the old hens you might consider red, gold, and black sex links (also called Stars or Comets). Generally speaking, they are slightly smaller (thus less meaty), eat less, mild mannered, and very good layers. Again, I would recommend good breeders rather than feed store birds. There are many breeds that can be crossed to produce sex link birds and the parentage will have an effect on the quality of the offspring. Large commercial breeders that supply feed stores aren't always that concerned about the finer points. It's more important to them to be able to simply sex the chicks accurately. Some people have gone so far as to say that the big breeders use poorer quality stock so the customers will have to buy new chicks more often. I have no idea if that's true.
