Ah, good point...I guess I missed that part
Propane fired outdoor cookers may be the ticket then...I'm thinking a double-boiler type of set-up would be a good way for reheating...having a high enough capacity for that much food, well, it may take some creativity to come up with a solution for that. If you can put enough smokers/grills on site for reheating with indirect heat is a very good option as well.
The good part is you have time to plan for it...spur of the moment gigs are tougher...been there, and it can take some of the fun out of it if you begin to get overwhelmed.
If you need a bit more help on figuring out what you can do with the equipment you have, let us know what you have to work with, and we can figure out your best options...if you have a good game plan that you are comfortable with, and can stick to it, you'll do fine.
EDIT: a generator and the electric cookers may be a viable option...as long as your generator's muffler has a spark arrester, it should be approved for Parks/Rec Areas.