You can buy virtually anything from Dillon one piece at a time.
I have quick change kits for my 650 on each caliber I load ...with dies, powder measure and the powder check die installed and ready to go.....380, 9mm, .40, .45acp, .38 spl, .357 mag and .44 mag....
Dillon dies come apart easier than some for cleaning.....and the bullet seating die and final crimp die are separate dies....some dies do that in 1 step / I prefer the Dillon setup.
But adding these things a little at a time are fine too....in Lynnwood area .... Precise shooters and Adventure sports both have inventory on dies and a few accessories and parts if you don't want to go direct to Dillon & wait for shipment. They often have a 650 in Stock too...