questions you need to ask yourself first;
what type of shots you want to create?
how many pax you will need to shoot?
if you plan just to have simple head and shoulder shot of individuals, one off camera hotshoe flash with umbrella set up is good enough, you need to have a light stand can rise the light to 2m high for more decent lighting.
and if you plan to shoot more than 50 pax, you will need a studio strobe instead of using hotshoe flash, as you will need to replace the weak batteries and let the flash rest for preventing overheating.
seem light you don't have anything except your camera, so get a cheap hotshoe flash, light stand, umbrella, umbrella adaptor, plus batteries will cost around $200.00, and that is bare minimum set up, assuming you already sponsor your service, if you spend another $200, do you think you can afford it? and use them in future? will it become white elephant after the project?
why not just go and rent it, you can spend less with more decent set up, it will make your life easier too.
either the organizer pay for the rental expenses or let sponsors get tax rebate for making donation on the rental expenses.