What should I Iook for when buying 12 new computers for my non-profit organization?

I’m looking into buying some desktops for my non-profit organization. My supervisor thinks we should look for DELL products; however, I have no knowledge of what would be required to properly operate 12 desktops. I mean, what speed should I be looking for? What additional gadgets or accessories should I include in my potential budget cost for these machines? We will be using the desktops to provide services for the community. People will come in and learn how to use the media/technology to organize on social justice issues, so we don’t need something super fancy, but we definitely want to get something good, something that will last us for several years. Does anyone have any recommendations? If you know of any organizations/websites that offer special deals to non-profits for technology I would really appreciate it. Thanks!