I think a Dutch GP can attract a great crowd, if it's held in the right place. The Gelredome stadion might be the place to be, close to the German border as well. Speedway and grasstrack is pretty much a shared sport in NL (also DE), most people visit both sports.
I can see many Germans, Poles, Danes etc. coming over to NL. The icespeedway GP's in Assen attract every year loads of visitors, I believe that there are more foreigners there then Dutch people.
Melbourne?