Another thing you might consider is what people in your area want. If the purpose is to raise chicks to sell
it does you no good to raise a breed that nobody wants to buy. If your area has mostly backyard flocks with folks just keeping a few birds for eggs than you would want to go with a breed that is known for its egg laying ability. You might also consider the friendly/flightly characteristics of the breed as many folks want a tamer chicken
in their backyard. Also how does the breed do with confinement, as the reality of many backyard birds is that they will be kept in smaller coops/runs at least some of the time.
If your market is going to be more of a rural area where folks have a bit more land
and are more likely to keep a free ranging flock then the breed you want to invest in is going to need better foraging skills and predator avoidance.
I prefer a good foraging chicken
and am happy to have dual purpose birds even if it means fewer eggs. My reasoning for this is that I have a property where the chickens
can free-range and I want the insect control
help from the birds. My bird do get a significant about of their food from foraging and that matters more to me as I live on a remote island in AK where shipping in anything is expensive (both in $$$ and environmental impact) so the less feed
i need to buy the better. In a farm region where grain is plentiful and feed is cheaper this might not be as big a factor. I would hope from a permie
standpoint that everyone would want as much of their food/their animals food to come from a local source. However if you're selling to a suburban population they might not be able to allow their birds to forage and may not want to accept the reduced egg laying from a true dual purpose bird if they're going to have to confine and feed it grain anyway.
I have mutt chickens, they're a mix of Wyandottes, Easter Eggers, and Black rock. Mine came from a local keeper who has had a free-ranging flock for years and sells some chicks and eggs as a small business. I don't care that they're mutts, I kind of actually prefer it as I know they've been breed to do well under local conditions, forage well, and are good egg layers. While it's great to want to breed and help conserve a rarer chicken you might want to consider if that's really the way you want to go.
You mention that you already keep a mixed flock of layers. If you add a breeding flock do you have plans
to keep them apart. If you go with three breeds do you have the setup to keep three (or four if you count the laying flock) separated? If not then you're going to end up with mutt chickens anyway. I would suggest that you invest in a good roo, from a good production line, with a good temperament. Looking at fairs and shows in your area is good advice. Let him take care of your girls to start and see how raising those chicks works out for you and your son. If he's still interested and the project
goes well you can always expand to keep a pure breed flock in the future or you might find a good mix that you can develop into a locally adapted flock. Selling a mixed bag of chicks might also work in your favor as many casual chicken keepers like have a few different breeds, different colored eggs, different looking chickens in their flock.