Is Diablo 3 Reaper of Souls standalone?

Is Diablo 3 Reaper of Souls standalone?

Yes, you need to buy both, unless it's for a console. Reaper of Souls is just an expansion and doesn't include the base game. Consoles have The Ultimate Evil edition, which includes both the base game and the expansion. You can play the original Diablo 3 without the expansion however and buy the expansion later.