Your lambs will be eating grass, hopefully, when you get them and well into the winter. They may need some hay or straw after Christmas when there is little nourishment left in the grass, until it starts to grow again in spring.
Up here we need to give store lambs a little cake from October onwards. I'm not sure where you are and whether yours will need cake over winter. Keep an eye on their condition - look up how to do condition scoring - and if they are losing condition, give them a bit of cake. Just 1/2lb per head per day will make a lot of difference.