1. I need a Thunderbolt Cable?
No. Apple Displays come with a cable attached.
2. Can I buy any LCD apple screen? even if it is second hand?
Yes. Almost any display will work (with the correct adapter).
3. Can I buy a package with a Keyboard and LCD?
4. Is a second hand keyboard OK also?
Yes if it uses USB or Bluetooth and has the Apple key positions.
5. Does the Keyboard plug into the screen or do I need to buy another cord to plug into the Air?
Apple displays have USB ports that can be used for keyboards