I am looking at integrating a payment mechanism into an app.
I have used stripe and Apple Pay and can successfully take payments.
My question is do all Apple iPhones support Apple Pay?
Is having just Apple Pay sufficient or do I need another system if Apple Pay is not available on some devices?
What is the best way of approaching this?
With regard to "Do all Apple Phones support Apple Pay",
The list of devices that support Apple Pay are listed on Apple's support page
Essentially, it is:
iPhone 5 and 5S are the notable omissions from this list (although I believe that iPhone 5S can be used if it is paired with an Apple Watch).
More importantly, just because the device is Apple Pay capable, it does not mean that the user has enrolled a card into Apple Pay (It may not be supported by their financial institution or they just may not want to use it).
As to "what is the best way to approach this?" - This is subjective and is off-topic.