Unless you (a) have experience and competence with Spanish and Mexican cuisines; or, (b) the clients insist, stick to what you know.
Spanish and Mexican cuisines are, by and large, very different. The way you lump them together in your original post could be a clue that you're not proficient in either -- or it just may be that you wrote in a quick sort of shorthand.
Cooking for 100 is not an easy task -- triply hard when you're doing it for the first time. The organizational, logistic, and management hurdles are very high. Don't let preparing unfamiliar food make it harder. Now is not the time to learn how to make mole colaradito or mole negro.
On the other hand, if Oaxacan is your thing, you couldn't do much better. And, speaking of moles, many other Mexican dishes lend themselves to large event catering. For instance, all of the various stews, tamales, rice, beans, flans, and so on. Spanish food, by and large, is more ad hoc and less caterer friendly -- at least without a large kitchen and staff. Not that it can't be done, but you have to make good choices.
If you do have a reliable recipe (and enough equipment) for an arroze, like arroz con pollo, it's not a bad idea. For one thing, the new and old world versions aren't all that different. For another they can be made in large quantities and/or batches; benefit from holding; may be held and reheated; may be reheated easily; may be held at serving temperature for a long time; and a bunch of other good things. But again -- if it's something you haven't done before, 100 covers is not a good first time.
Ex owner operator of Predmoninantly French: Intimate Catering; ex cook at a couple of good joints