In a simple quick answer to your question:
-Stripped Lowers - the only piece you need to register
-Lower Parts Kits - comes with the pistol grip and all of the internals (fire control) of the lower.
-Complete Upper - may or may not include the BCG and charge handle.
-Stock - any you want
-Bullet button device unless u want to go featureless.
Consult this for legality issues consult the flowchart in the tabs above.
And here are some sources: