While you might luck out with unusual ammo. I've heard stories of people going out for hunting season and all of the 30-06 ammo being gone because its popular, but 8mm for his rifle luckily being in stock because he was the only one shooting it. However, in general, ammunition for common calibers will be commonly available. 22lr may not be sexy, but its common as dirt and almost as cheap. The same goes for 12 gauge, 30-06, 9mm parabellum, 45 ACP, 38 special, to a lesser extent 20 gauge, 357 and possibly 30-30. I'm sure there are others, but these I can think of off the top of my head. Other rounds may work better, perhaps much better, but these will work well enough, and you can almost always find them. When ammo isn't short, you can find them cheap and stock up. A few hundred rounds is enough, you don't need to prepare for a siege. If you shoot enough you need more, well you'll know better than I will and you can always learn as you go.

When it comes to teaching marksmanship fundamentals and hunting small game, 50 to 100 yards is plenty of range, and 22lr can easily handle it. I hunted rabbits at something less than 10 yards with a 22 pistol. I could have taken quail or squirrel just as easily.

This isn't to dissuade you from a less common caliber weapon, but its nice to be able to find ammo, magazines, repair parts and in the case of pistols holsters. If I ever bought another pistol it would be a Glock just because you can get anything you want for them anywhere that sells firearms.