The game has an insanely lenient buffer system. Most moves you can mash out really quickly and the character will follow through with the entire thing, but there are some exceptions in the game. Mostly the VF characters.
Depends on what do you mean by combo. DOA doesn't have real combos, the only guaranteed damage is from juggles. And yes the timing is very lenient compared to VF or Tekken --- I have a radio in my car.