To my mind there is far too much in your post. What you are proposing is a complex project that needs to be approached as a series of small separate parts - for example reading or writing NFC tags, or sending keystrokes. Don't try to develop a complex project all at once.
Also, don't attempt to buy all the parts up front as you will probably change your mind as your knowledge grows.
If you can identify a particular part of the project that you need help with then it will be much easier to give useful advice.
You mention using an Arduino to send keystrokes to an EPOS device. That seems like a strange way to do it, are you sure it is possible for the EPOS device to receive keystrokes?
If you want to send keystrokes then you could use a Leonardo or Micro or AFAIK a DUE - but I have no experience with a DUE.
Another thing to consider is whether it might be easier to implement your project with a RaspberryPi which is much more powerful.
...R