These are one of the oldest varieties of Sussex fowl, and were traditionally dual purpose Farmyard foragers, which later were fed on to produce fat table poultry for the London market.
They aren't as prolific in the laying stakes as their Light or White cousins, and won't come close to your hybrid layers in production either, but they will certainly score in the looks department!
Sussex are undemanding, hardy birds, not prone to excessive flightiness, and should settle in with your hens after careful introduction. Do you know if they are immunized against Mareks disease or not? Most hybrids are as day olds, but many/most pure breeds are not, which can mean unprotected birds are at risk from vaccinated birds, that will be carriers of the disease. Having said this, Many breeds of poultry have a natural immunity to most strains of Mareks, and as far as I know, Sussex as a breed are not Excessively prone to this disease.