If you know Dario Argento, you know this isn't for kids. Although the first murder isn't quite as bloody as his usual fare.
Yes, the protagonist doesn't look like much, but he's supposed to be similar to a Hitchcock protagonist. Those were usually ordinary folks caught up in unusual (for them) events. In this case, apparently a film student. Although he has less reason for his involvement than Hitchcock's characters ever did, as far as I can tell.
Not a fast-paced movie, but the speed's appropriate for this type of suspense storyline.
He was tailing her, not chasing her. In a chase, she'd be actively trying to lose him, not going to work.
I agree that the conclusion is rather weak. Julio decides on one person's guilt based entirely on video rental patterns. And there's no hint of the true killer until it literally comes out of nowhere.