What your aircraft can do wrt to RNAV/RNP ops has less to do with what equipment you have and more to do with how the aircraft is certified. This will vary from tail no. to tail no. Two aircraft with exactly the same eqiupment may be certified differently due to manufacture date and requierments of the original operator.
It's a combination of what equipage you have and what it says in the AFM. Airbus for example will charge you $90 000 just for the piece of paper in the AFM that says you can conduct RNP-AR ops down to 0.3nm in an aircraft that doesn't need any equipment upgrades.