Denti:
Tuning an ILS does NOT "prevent IAN from doing its thing"! The I in IAN stands for "Integrated", and the system chooses the best available NAVAIDs for the approach. If an RNAV/GPS approach is loaded, and an ILS is available for the same runway, the system simply uses the LOC and G/S for guidance, once captured, since they are more accurate than LNAV and VNAV.
flying800:
Just think of IAN as integrating the Approach Mode into RNAV/GPS approaches in addition to ILS approaches.
Limitations and authorizations depend on the specific airline. In general, you can eventually expect the same RNAV approach to DH, and Cat II/Cat III authorizations the airline already has.