What flavors of ice cream? I'd do the basics: vanilla, chocolate, strawberry and maybe a few more popular ones, like cookies & cream, chocolate chip, etc.
How much to buy? I would plan on a few scoops per person, so just do that math and go from there.
Toppings? Whipped cream, sprinkles, syrup (chocolate, strawberry, caramel), nuts, crushed cookies & candy, fruit (go to any Marble Slab or similar ice cream place for fun ideas!)
Cones or just bowls? I'd do both, though instead of regular bowls you can buy waffle cone bowls. Portable, no trash, and yummy!
Drinks? Water, soda, iced tea, lemonade