That sounds great - keep it big (i.e. enough of one item) and simple. Although - do you need a veggie option? How about some kind of bean salad, which will also be fine for vegan/dairy intolerant etc. I do one that is: 2 cans of kidney beans (rinsed), 2 cans of sweetcorn, a few chopped peppers (into little cubes), drizzles over olive oil and the juice of one or two limes and some coriander. Mix together. Dead simple and looks very colourful.
You could probably double those amounts for 100 people as the hardcore meat eaters probably won't touch it with a bargepole!