Erin, I've kept hens since 1959. My mother and grandmother taught me and meemaw learned from her mother in the 1890's. Things were much different back then. Unless you kept somewhat higher strung Leggerns', as they called them, you probably kept a dual purpose bird precisely because you needed dual purpose birds. Life was tough for my immigrant, agrarian grandparents and they were self reliant, peasant farmers.
Today, you have more choices. The "meat birds" development means you can raise out meat birds in as little as 8-9 weeks. Done. The other end of the spectrum has seen the poultry genetics companies breed ever more specialized layers. These are early maturing, huge egg laying, great feed conversion, light weight birds who will lay up a storm for a few years, but who's carcass is very thin. These are the commercial red sex links, specked white leghorn based hybrids, etc. The intention of the producers of these birds is that you'll be back each spring for another dozen pullet chicks while a dozen older layers are going out the back door of the coop. Nothing wrong with that system, just saying, that's kinda the way it is designed.
The dual purpose birds have also changed, however. The birds being sold today only somewhat resemble their fore bearers from the 1920's. A hatchery barred rock or hatchery white rock isn't really the same bird after 100 years of hatchery breeding. Virtually every strain of hatchery bird could now be called something of a "production" strain. They are laying more than the birds of the same breed did years ago. The coloration, body type, feathering, temperament, ability to brood, etc, is far different than those same breeds possessed in great grandma's day in the early 1900s.
I do believe that you can take a 40 bird flock of hatchery stock and be very, very selective and in a few years, you'll likely have a stout flock of healthy, stronger birds. Or, you can start right off with a few breeding trios of heritage stock and continue on with them. I'm not talking about showing, per see, just strong, healthy, productive birds. In my view, you'll have to become accustomed to heavy culling and strict selection. If not? Just getting 40 hatchery birds and allowing them all the offer eggs for the incubator will produce a lot of mediocrity. Well, I've gone on too long.