If you are buying in the queen then the answer to how soon you can re-queen is how soon is the stock available from the vendor?
Cart and horse here. In right order preferably.
As for how much brood your colony has then to my mind the more the merrier, and I too would bank your "scrap" queen as in insurance.
It is widely written that it is easier to introduce the same strain to same strain.
If you are thinking of one of the more "exotic" types of bee then also consider who in your area keeps what as their drones are going to have an influence down the line whether you like it or not.
PH