Actually you can use either NPN or PNP sensors, it's just that you need to wire them differently.
There is a quick start guide to identifying your sensor and how to wire it.
Some sensors may require pull up or pull down resistors while others have them built in.
The question isn't how hard it is to convert a particular machine to MASSO but how hard is it for you to convert the machine.
What you find easy I may find hard and visa versa. It doesn't make the task easier or harder, it remains the same.
I could venture an opinion that MASSO is no harder to install than any other CNC controller and probably easier than most but that is my personal opinion.
I suggest that you start by researching each component on your current machine.
What type of drives do you have and how will they wire?
Get wiring diagrams and manuals for each component so you know how they work.
Do the wires all come to a common point or will I need to rerun them?
What do I need to add and what can I re-use?
What model VFD do I have and how will it connect? Get it's manual.
What sort of tool changer do I have and what is its logic requirements? Will it work with an existing logic?
Make copies of the axis settings so they can be transferred across to MASSO as this will save a lot of calibration work.
Work on each component and once you have the answers to these questions you will know if it will be easy or hard for you.
If you do the research and prepare it will probably take no time at all and will be easy.
It probably doesn't answer your question but easy and hard is personal and only you can answer this question.