you will need a holding number (aka CPH number) for the land, so you can keep livestock on it. Then you will need a flock number for each species you get in, tho they may use the same number for second and third species you still have to tell them as they then send you the relevant regulations, register and keepers guides.
Get the holding number straight away you decide you are going to keep animals, get the flock no as soon as you are planning to go somewhere to get some animals. Dont wait til you have them, they dont like that.
'They' being your local animal health office. not sure where you are but easy to find out from council etc or google.
Costs for slaughter for us (albeit in N Scotland) are £17 a sheep, and butchery is approx £15-17 per sheep.
Good to stick to friends and family as if you are going to sell it, it has to go from abbatoir to butchery and butchery to customer in refridgerated transport, and if you dont have that it can add to the cost a lot to pay for this.
Market prices will depend on age of sheep, and breed and condition. eg my unreg shetlands might only fetch £20-30 but a big continental like texel might fetch £90!! so need to work out what breed you want, which will depend on what quality land you have and how much, whether you are OK to shear them or need a non shearing breed, whether amount (continental breed) or quality(native trad breed) of meat is critical, whether you need one small enough to handle solo, whether you need one that is easily contained by your fencing etc.