I would not shell out any money on equipment unless you have in your contract a guaranteed number of print sales that will justify you profit along with your hefty expense. Who will be paying for the prints, are they extra costs for the attendees, are they complimentary by the charity to get the attendees to open their wallets for donations and if so does the amount of money you are receiving from the NGO justify the extra expense? Can you rent the printer from a local rental house or see if your local lab can send a printer and operator to the event with you?
I know a lot of questions, but I have been talked into doing these charity events before and they are neither profitable nor do they generate true leads. I've also had some very honest insiders insight into NGOs and not only DO they have money other venders charge them premium rate while at the same time scamming artists with their we're not in the advertising business we are charity.
I know it sounds like bad business advise but it is ok to let the client know you are not going to deliver beyond your means and what they are paying for.