Start with three hens - they lay best in their first season, so if you start with ten for example, you'll have to get rid of them all at the same time. If you're happy to do that, fine but you may want to get say three a year to replace those that "fall off the perch".
You can eat spent layers but there won't be much on them - good stock for soup. For eating you either want surplus cockerels (if you hatch eggs, you'll always get cockerels) or meat birds.
To be truly organic in terms of marketing, you have to be registered with a certification body like the Soils Association. For most people, that's not an option (too dear, too many rules). As previous posts say, you can buy organic layer pellets and organic mixed corn OR you can buy GM free versions which are cheaper. Your call, really.