As an American, the "down" slang sound weird.

The most I've heard in the wild is a short "I'm down.", you wouldn't say "I'm up." (unless you're saying you woke up, or you're high, or your gambling is going well)

Other than that, your versions are more common in the US as well, from my experience.

edit: Thinking more on it... "I'm up for that" would be used if you were slightly hesitant. "I'm down for that" implies a lot more confidence.