First, make sure you, and anyone you hire can really cook spanish ethnic dishes.
Most people have an affair catered because they don't want to cook the food themselves, not because they cannot cook. It's more about quantity work. Cooking for 25 people is way different than cooking for 4.
Go to 4-5 restaurants and see what on most of the menus. Hispanic ethnic cooking is big on chicken, beef, legumes, rice, corn, potatoes, eggs and flat breads like tortillas (corn or flour). Most dishes are not chock full of vegetables because many locations have a climate that is too arid to sustain above ground , delicate, vegetables. Tomatoes, peppers, onions and corn are the widely used vegetables.
Once you visit a few restaurants, buy a couple of really good cookbooks. Get authentic ethnic cooking books, not something that some bogus television celebrity chef put out. Then, start practicing. If you are not hispanic, yet want to cater to that community, your food has to be as good as 'abuela' made. Remember, the idea people have the party catered is because they don't want to cook, not because they cannot.
I'd have 4-5 light, first course/appetizer type dishes
10 main entrees, 2 vegetarian of the 10
3 rice varieties
4 side dishes, 1 vegetarian of the 4
5 desserts