When you play poker, do you show all your cards to the other players?
Do you like to watch a movie whose plot you know in detail beforehand?
If you are a GM, YOU are in control. Some people like to let their players read a campaign or scenario description, like to let them decide whether they want to play it, but you should not let your players create the scenario. It kind of spoils the fun.
Include what you deem appropriate. Have a plot ready a long time before you launch the scenario. You might build you plot on the "Independence day" principle (based on cool scenes and player expectations or you might base your story on plot quality (novella approach). You should not GM a story the players created.
As for Jinx's question: Adventurers is a broad category. I guess it'll suffice. More precisely you could have answered "I need warriors and thieves" or "I need an Ulmanian Priest and his party of Ulmanian Grotahn Sect Warriors".