Hi,
If the AP is of type "IAP", i.e. IAP-315 then the ap can run the Instant OS as well as ArubaOS. If the AP is of type "AP" i.e. AP-315, then the AP can only run ArubaOS. This does change in some of the new AP's, as Aruba is moving towards the concept of a "Universal AP" that can run both.
If you obtain the IAP type AP's then they will ship with the Instant OS running, you will need to connect them and then convert them to ArubaOS.
there is no difference in licensing for this.
you will need the AP license for the mobility controller equal to the number of access points. (as well as any other licenses you might want i.e. PEF/VIA/RFP/ACR)