the difference between commercial meat birds (CornishX, and even - tho to a lesser extent - more 'natural' ones like red broilers or 'colored rangers' or etc) and dual purpose birds is not just growth rate, it is the type of carcass you get and how much it costs per lb.
Dual purpose birds are not really 'dual purpose' except in comparison to, like, Leghorns. You don't get much meat off the carcass, especially little breast meat, and it costs a lot more per lb to get that meat than if you'd raised commercial type broilers.
I am not in any way knocking dual purpose birds, since the meat you DO get is much TASTIER. Just, be aware.
Anyhow, a good (non hatchery) RIR would work well, or NHR although by reputation the roos are more apt to be nasty; or a good (non hatchery) rock or wyandotte roo if you do not care about the purebredness or color of chicks.
Have fun,
Pat