That's the great thing about Scooby, kids of every age can enjoy him! Believe me, I've already spent more time playing around with these sets than my kids have.
Regarding your daughter's costume, will she be flying solo, or going with a group of friends dressed as the whole Scooby gang?
My wife and I have dressed as Fred and Daphne, as well as Shaggy and Velma on different years for our couples costumes, and we have gone to conventions as the whole gang with the kids. Once we were at a show and I was Shaggy and my wife and both girls went dressed as Daphne, the people there got a real kick out of it. We have even gone out as the Scooby team dressed as the Ghostbusters.
The girls always wait until the last minute to choose their costumes every year, and sometimes they wear one costume to school in the day, and a different costume to go trick-or-treating. By now my wife and I usually have our costumes picked out already but we have been so busy lately that we didn't do it yet, the same thing happened last year and we had to rush to find good costumes.