"Wouldn’t an iPad and AOA system meet all the requirements?"
No it would not. Portable navigation devices do not meet TSO requirements.
I have flown IFR in a 185 on floats no transponder with a single nav com. Yeah it can be done, not easy but possible. I have flown 6 legs in a 737 without an autopilot and I shot an approach on 3 of those legs. Legally it can be done, not easy, but it can be done. Point is this. Every mission is different and every mission requires an evaluation of what you are going to do, what is the required equipment for the mission, is that enough equipment to safely execute the mission and finally are you proficient enough to use that level of equipment. Nor is the required equipment sometimes clear. For instance, yes you can operate IFR with an approved GPS only, and you can shoot a GPS approach at your destination. HOWEVER if your destination requires an alternate (failure to meet the 1-2-3 rule) you MAY NOT predicate your alternate minima upon the GPS approach there UNLESS your GPS is WAAS enabled and approved under TSO C145 or C146. However the alternate approach must be based upon GPS LNAV only, or Circling minimums, not VNAV or LPV . That is not in Part 91 but is in the AIM and the approval certificates for your equipment. That is one example of "navigation equipment appropriate for the route to be flown". Thus if you were fly a mission with only a GPS approach at the destination airport (or all you had was GPS), and you had an alternate required and you could only execute a GPS approach there, if you are in a non WAAS situation either by equipment, NOTAM or RAIM, you cannot complete the mission. Careful evaluation is always required of the mission well beyond the regulations in FAR 91, 135 and 121 but down to the certification basis of your equipment and aircraft.
