Swaping disk brake wheels?


i want second pair of wheels with disk brakes (my bike specs allows to use 700 and 650)

I see that store sell you only wheels, what about disks? Are'n they attached to wheel?

Ideally, i just want 2 pairs that i can switch without any adjustments, rear one with it own cassette. 

So do i need to buy disk separatly? Or disks could easily swappped, without adjustments?

Would i have to adjust deralers each time i swap wheels?