Only the original is worth playing. The sequel is a significant downgrade in most of the little details that make up the whole, and it takes steps away from what made the first game stand out. Do not believe people telling you that Off the Record fixes anything.
Most of the difficulty is in how much you choose to take on, completing the main story missions is not that big of a challenge, but if you are also attempting to save every survivor then you might be in for some seriously frustrating fun. Herding finicky survivors that are under constant threat from the sea of zombies while you are under heavy time pressure is super fun. Everything being a weapon plays into this, the open world is a toolbox and you need to learn it well to get to where you want with every idiot alive. I say its important to play it without looking up where you can find various powerful weapons, learning where to pick up the right equipment and the best way no navigate the map is an enjoyable part of the experience. The game becomes quite easy once you have mastered this and the survivor AI.
The game has many endings and your character progression carries over between playthroughs, so there is no need to rush on your first time.