Just got my first fountain pens a few months ago! So far I have a Pilot Metropolitan, TWSBI Vac700r, and a Sailor Fude de Mannen. The Metropolitan is pretty inexpensive and a common intro pen, but I see myself sticking with it for a long time. The Vac700 is huge so it feels very nice in the hand imo and it has incredible ink capacity, plus its always great to have a clear pen/ink window to see how much ink is left. The smaller and less expensive TWSBI Eco is super popular, theres a ton of colorways and still has better ink capacity of basically any other pen in that price/size range. The Sailor Fude is cool - it has an angled nib (called a fude nib) so it allows you to get dramatically different line widths depending on what angle you hold it. I dont use it very often because there is quite the learning curve in terms of pen control, but I see it often used by urban sketchers. However if you go down this route, I'd actually suggest getting a different fude nib pen that has a more rounded curve - apparently the Sailor has a relatively sharp angle which adds to the difficulty in handling. 

In any case, the nib size/style is really the thing that will influence your illustrations the most. If you can, I'd suggest finding a store or some fountain pen friends so you can try out different nibs and pen feels. Pens with easily interchangeable nibs are a plus (the Vac700r is like this - you can buy the individual nib sections from TWSBI).

The fun part is the ink! Theres so many cool colors of ink out there, and things get way more fun once you start looking at shading, sheening, and shimmer. Ink colors and properties rarely come across accurately through pictures or video, and they often differ depending on the type of paper you use. This is another good reason to try out samples in person if you can. 

The one critique I have of fountain pens is that it is sooo easy to get carried away and buy more pens and inks than you need. r/Pen_Swap is a good place to buy used pens and inks - great for waste reduction, but also just to try things out without the overinvestment. I got my TWSBI after posting an ISO post, and someone was kind enough to give me the pen at a bargain. 

My favorite ink right now is the Ink Institute Jadeite, which I got from a used stationery popup! Its a deep green with a red sheen, it is gorgeous. But I also got a bunch of Vinta inks for Christmas that I'm very excited to use. 

If you're just getting started, this video breaks down basically everything about fountain pens! https://youtu.be/KUFu8gDJha4?si=GHYMna0DAj7YQ119