LOL, You have two overlapping topics: (1) minimum tools needed to make a basic pen. And (2) Kitless, tap/die, fountain pens and rollerball versions. Yes, I clumped a bunch of topics under #2.
I will suggest that #1 is pretty straightforward; you will get a variety of good suggestions, but they will wander based on experience and preference.
#2 is not a 'basic' skill set topic, so it would be helpful (in my opinion) to get through #1 before #2.
OK, so my humble experience has led me to gravitate to:
3/4" Roughing Gouge (Sorby)
2 parting tools - 1/8" and 1/16" (Sorby)
Magicskew - all cutters
Easy Wood Tools - all cutters
Sorby Red Handled Micro tool Set.
3 set diamond hones
I probably have 10-15 more gouges/tools that I have rarely used... After 12 years, these are what I use 95% of the time.
I have never made a true kitless pen, used taps and dies (for pens

); so I cannot offer suggestions for #2. I suspect others with better experience will chime in. Be well and have fun!