Not at all naive. Single breed chicken keeping doesn't seem to be that popular in the US but it's quite common with serious chicken enthusiasts elsewhere.Getting ready to find some new chicks and build a coop while they start to mature. Seems like it would be nearly impossible to prohibit interbreeding without a lot of work, I want to find the right breed of dual purpose bird. I don't want to cross-breed anything because I want the natural traits of the birds I choose to remain intact. We want brooders that produce a good amount of eggs but also, eventually meat.
I want one breed because I do not want to depend on a hatchery to provide brooders for my egg layers, etc. If I have more than one breed of rooster, there will be no way for me to know which rooster has fertilized which eggs, etc. My ideal is to buy chicks once, and create a sustainable flock from those chicks.
From what I've read, Orpingtons seem pretty perfect. Forage well, generally too big for hawks but relatively quick on the ground for their size, 4-5 large eggs per week, great brooders and mothers, and good size for eating. Still researching breeds, though.
Regardless of which breed I go with, am I naive in thinking this can be done?
You have already identified some of the advantages. You are likely to find more as you develop a sustainable flock which I won't go into here.
I wouldn't worry much about cold or heat tolerance. It's bit of a nonsense when you look at what little evidence there is. Within the temerature range you wrote most breeds will cope.
A few points I feel are worthwhile pointing out.
There is no chicken too big for hawks. The hawk risk depends on the type of hawks you have in your area.
To preserve the natural traits of a breed they need to be there in the first place. This will be a major problem if you buy chicks from a hatchery. You would be better off finding a reputable breeder, paying them a visit to see how they keep their chickens and make your own assesment of the breeders flock health.
What many people I know who are aiming for long term sustainable closed flock keeping is to buy a breeding pair. or two at around one year old. At a year old you can tell a lot more about the bird than you can when they are chicks and hopefully the chick type health problems will have already killed of those who fell sick.
What you don't write is how you intend to keep them and this is far more important when trying to decide on a breed.
If you are planning on any type of free ranging then foraging skills and predator awareness are more important than egg laying ability. Dead hens don't lay eggs.
If you're going to free range then the getting a breeding pairs from a breeder who free ranges their birds and broody hatches is really important. Yep, good breeders still do this rather than use an incubator. An incubator hatched chick and human reared doesn't know any of the things a mother hen teaches a chick and the probability is you will get more losses while they learn, if they learn.
I would consider some of the less common breeds.