How old are they when you buy them? Are you fattening them over spring / summer or will you have to buy hay as well? Will they be grass fed or shall you cake them? Do you have the grass or are you paying for grazing? You might need to give them some minerals and worm them, maybe vaccinate them.
Kill and Butchery will likely cost around £25 - £30/head depending on size, plus extra for making sausages, burgers etc.
They should be a decent size, expect something like 20kgs - 22kgs meat per animal maybe? (Before boning and trimming.)
Would you sell as boxed half lamb? What price do you think you would get?
If you don't have sufficient local market who will pick up from the butcher or from yourself then you might need to add costs for packaging for mail order. Getting set up to do mail order meat is expensive, largely because of the carriage to get the materials to you.