Human Rachael and HUNK have the best buffs in the game. You can get the former fairly early in Raid Mode, the latter towards the end.

That said, I don't see myself buying this DLC, especially not for $3 each. Ooze Rachel loses Human Rachael's magnum and shotgun buffs in favor of ridiculous physical attack/melee buffs. Not sure what Lady HUNK has. Ultimately, they don't seem worth it. If there are additional DLC Raid Mode characters in the future, maybe Capcom will offer them at a discounted set. I just can't see myself spending so much on a single character. Also, I'd be far more interested in more enemies and stages which doesn't seem likely.