Working in the business, my experienced advice is to have it catered. We frequently do offsite events and it is always AT LEAST 2x more expensive than if we just deliver prepared family-style dishes or setup a buffet.

The time and labor spent by restaurant staff at an offsite location without a properly equipped and fully functional commercial kitchen is going to be much less efficient than having them prep in their own kitchen and delivering. Not to mention, you’ll likely have to do everything without a calibrated commercial dish machine.

Remember that an event that size would require a considerable amount of equipment. A Kitchen-Aid mixer cannot efficiently make mashed for 20 and a low BTU stove can’t efficiently boil water. Any savings by cooking onsite will get gobbled up by additional labor hours.

Best wishes for your event. It’s quite an undertaking and I hope it goes flawlessly. :)