.40 S&W is a solid handgun caliber, Can't really say more without knowing what brand you're getting. 9mm and .45 ACP are more common rounds though
Ditto to the AR-15, it's a good platform but I can't really comment without knowing more about it, e.g. bolt carrier group, barrel length and make, caliber etc.
Mossberg is the way to go for shotguns, a 500 or a 590 will do just fine, 590s are functionally the same as 500s but with a slightly thicker barrel, either will work just fine