"Is buying DRM-free songs from Amazon integrated yet?"
Will be as soon someone writes Amazon service browser.

"Will it support playing video/music video? (show in the playlists and just call up the appropriate application to handle it)"
Amarok will play (internally, no need for external apps thanks to Phonon) any videos manually added to the playlist, but nothing more (no collection, stats, etc). At least that was the plan back in the day.