Black Australorps hold the record for most eggs laid in a year, and mine lay well and are easy to handle. Or get two different breeds, so you can easily tell them apart. I agree, definitely two hens -- I love my roos, but one roo with one hen will mean the hen will get overmated. As for chicks or older chickens, just get both the same, whatever you get. Mixing ages has its own set of difficulties, and may well mean keeping them separate for a few months, and would also mean chickens living by themselves, which is also not good as they are flock animals.
The catch on older birds is, people are prone to selling off their older birds who have slowed down or even stopped laying, saying they are younger than they are, so it depends how well you know the source. Once a chicken is full size, it's virtually impossible to determine age.