I'm still fairly new to making and using Fountain Pens myself and I see several similar recommendations for good pens, but I'd like to take a different slant on the choice if possible.
What is your favorite rollerball pen currently? One that feels good and comfortable for you to hold (and that you likely have all necessary tooling for). Since most rollerball pens have a fountain pen equivalent, that's the one I'd recommend you get for your first FP, and try it out for yourself for a while to get the feel of it and learn about them. If this happens to be one of those mentioned above, that's great but if not, stick with one you know for now.
This way your learning on a model you're comfortable with holding already and will only have to learn the nuances of withing with the FP nib instead of Rollerball or Ballpoint.