If I were to buy my first pistola I would get either a 9mm or .45. I believe these two to be the most popular cartridge world wide. Ammo should be easy to acquire almost anywhere. Next I would go for a .40. The .40 is good because on most platforms all you need is a .357 barrel to convert the weapon to the .357. During the recent ammo shortage, 9mm and .45 was more difficult to get ahold of but .40 was prevalent.
"Anticipate stupidity and stop it"
Be Kind, for everyone you meet is fighting a great battle.