A few tips.
TL;DR The Lamy Safari is the best first fountain pen.
- widely available - online from pen shops, in stores, at art supply stores, at specialty pen stores, all around the world
- colors - it comes in a variety of colors, so you can find something you like.
- cartridge and piston filler converter - although they usually come as a cartridge filled pen, you can buy a reasonably priced piston filler insert. The insert is made specifically for the pen, fits great, and works great.
- longevity in a pen collection - you will likely want to keep this pen around. the sytling can be fun and nice, and if you are traveling in an airplane, it is nice to have a cartridge filled pen as an option. Although, I typically bring a ballpoint in those cases.
- the pen body is slightly narrower than a wide body pen, but is pretty typical. similar in diameter to the typical pilot G2 gel pens that everyone seems to be using.
- designated finger position - the pen can be held in one position, but this situation is not a con. You need to learn to hold a fountain pen in a similar position because the nib writes in a specific position.
Avoid cartridges, opt for a piston filled fountain pen.
Cartridges have some advantages, especially for someone just getting in to fountain pens. However, long term you are going to want to fill your pen from an inkwell or bottle. The nice thing about cartridges is the initial simplicity.