I heard FO2 has more content than the first one. Can I play it like a sandbox game?

kind of...

you can pretty much do whatever you'd like in FO2, however it has a time-limit that really directs you toward completing the story. Although, there is a means to dramatically reduce your travel times in game and allow you to have more time to "sandbox" around in the world.

Fallout 2 does not have a time limit.

Although Hakunin the shaman will appear in various stages of degradation in the visions, this has no impact on the game.

You will find that Arroyo's fate is inexorable.

You're both kinda right/wrong.

Fallout 2 does have a time limit

IIRC its about 13 years.

I'm not aware of anyone who's come anywhere near it intentionaly.