1. Only meeting the minimum system requirements probably would not allow you to run X-Plane like you see in videos. Your computer will need to meet or exceed the recommended specs to fully utilize the rendering engine, and that does not take into consideration how demanding add ons can be. Detailed or un-optimized third party scenery and aircraft can also greatly affect X-Plane's performance.
2. The payware market brings more detail and variety to X-Plane. A lot of time third party devs make planes that are not included with the default simulator, or scenery that is much more detailed. KATL is one of the major airports though, so we have provided a fairly detailed default airport for that area.
3. That is up to you. Major stores include x-plane.org (no affiliation with this site or x-plane.com) or Aerosoft for example.