Do you have a recommendation of a younger manchego or one aged longer? I know a longer aged cheese is more flavorful, but it might not have enoug...

I think you will have a much easier time working with a younger Manchego or any of the mixed milk varieties (they have brand names like Campo de Montalban or Queso Iberico), anything older with a high proportion of cheese to flour will probably separate to some degree and leave more grease when baked.