Hi Sally,
A light is useful if you want to keep your girls laying through the winter. We just give ours a break though, as it just feels more natural. If you do install one, you'll want it to come on before sunrise rather than after sunset (that would mess with their little brains too much, since every time they went to bed it would get light again).
Also, Rosemary's advice is sound re not having all your birds growing old at the same time. However, chicken politics is pretty brutal at times, and introducing new members to the flock can also be tricky. Swings and roundabouts there, but it's best you know both sides.
HTH!