Define "established Superhero."
In Spec he had a whole summer of experience by the time of the first episode, as I recall, and in Amazing Friends and the MTV show he was already fairly active as Spider-Man.
Avengers: Earth's Mightiest Heroes also had Spidey as an established Superhero that SHIELD was interested in before he made his debut (with the infamous switch between Josh Keaton and Drake Bell).
There's also Spider-Man: Unlimited and most video games.
Even in the Ultimate Spider-Man cartoon Peter being the hero with the most experience is why Fury put him in charge of the team.