Looks great having just finished up almost 90 hours in a C-12V sim with the PL 21 pit. I know there are differences between the models and I can see a few different switch positions, which is expected for civie King Airs. You can do everything with one FMS, we actually have 2 in the military version and of course some mil only radios/navaids (Tacan/HF/UHF etc).
As long as the gauges support the full PL 21 suite (at least the basic required for flt pages), then LPV and LNAV/RNAV approaches can be done, FSX doesn't seem to care and I'm sure there is some way to trick the game to have the RNP levels needed. The real PL 21 autopilot will fly the approaches fine. The Navigraph subscription is workable for those that want to purchase the updates.
Understand the WX incompatibly issue, but do suggest at least working on the TCAS II and EGPWS systems, those are out in freeware form (basic versions). The ESIS we use can display Alt/Hdg/Aspd and Nav1.