The land doesn't need to be flat, but it does help with base stability. The short and simple explanation is: the more parts you are from the ground, the closer your base is to falling apart. Pay attention to the color of parts when mousing over them with the building hammer. Blue means it's grounded. Then it goes from green to red as it becomes less stable. You'll find more stable building materials as you progress.

Yes, later on you can unlock teleporters, but they don't allow some materials like metal ores and other specific items through.