The reason farmers get rid of their hens at 18 months old is because they are past their best laying days. You may get a hen that has used up all her egg supply, or they may go on to lay for another year or so.
They only have a certain amount of eggs and once they are gone, they are gone. Saying that, although our warrens aren't ex battery, they are the same breed, and they are 4 and a half and still laying a bit.
If you want to buy sexed chicks, you are better off with the hybrid breeds like the warrens, ISA's, black rocks etc, or other auto sexing breeds like cream legbars.
We've had ex battery in the past, and they came home with us and went straight on to layer's pellets. When I was told that we had to do special things for them, I called the rescuer we got them from and he said no, they will get used to the layers pellets, although they have been used to layers mash.
A bit of poultry spice in their feed will help give them a boost, as they will be in poor condition.
I would say though, if you do go down that route, don't expect too much from them, if you want them for eggs, then don't do it. Some may even die from the shock of being introduced to a normal environment after being confined like they were.
Rescuing ex batts is still popular, because people want them to have a loving home at the end of their days rather than be culled when they are in their prime of life.
But like I said, if you want them for eggs, then go for either chicks, or point of lay. Shop around though, there are a lot of cowboys out there charging way too much!