Pleasure, but you really won't NEED a 61 key midi controller. I use an old nanopad I bought from ebay for a tenner, which sits on the corner of the PA4X and runs all the transport controls in Reaper just fine. Your Nautilus or PA4X will work just fine as a master keyboard with the Native Instruments plugins. OK, it won't be as straightforward as with a dedicated midi controller but it won't cost you a penny and it will give you a chance to look for good deals, and decide what you really want. Good luck introducing yourself to a DAW. Youtube is your friend, there. I use Reaper because it's amazing value, and the user community is enthusiastic and very active: there are many many video courses on creating all genres of music using both midi and audio input in Reaper. And also I enjoy solving puzzles, which you need for all DAWs in my experience.
In fact if you are going down this route, having the nautilus already, I would suggest you start with a daw, and some free vst instruments, and learn how to use those. I wouldn't buy a PA4X or a controller keyboard yet. My guess is that once you have learned to use a daw and worked out your own "workflow" you will gave a greater sense of where you really want to spend your money. No one solution covers every situation, unfortunately.