Website build with CMS for charity

We are a small charity and we work to help street and slum children in Calcutta. We are working with one of the biggest ad agencies in London who are providing their time pro bono to re brand the charity. We need a new website building with a CMS so that we can then update the content inhouse.
WE CANNOT PAY FOR THIS PROJECT as we are very focussed on our income going to beneficiaries but this project would involve working with the ad agency and potentially open a few doors.