Group buy: Classic model. Would allow for more flexibility in kitting - the current kitting is optimised for preorder/in-stock
Preorder: Production is guaranteed, but there would still be a wait time before receiving the product. (e.g. preorders open while the set is in production, and the set needs to finish production and be shipped to the vendor.)
In-stock: Would be my preference, but requires high levels of interest in the set to convince vendors.