Get meat by the pound from the best taqueria nearby. For 20-30 I'd do about 12 lbs. Some asada, some carnitas, maybe some chicken or al pastor. Get onion, cilantro, and a bunch of salsas for taco toppings.
If you're going full authentic, cucumbers and jicama with lime and chili powder on the side.
Chafing dishes for tortillas and the meats. Also get Mexican sodas or aguas frescas, and you're good to go.
For reference, I've paid anywhere between $6-14 per pound. Carnitas are usually the cheapest. $10+ should only apply when their stuff is really, really good.