As someone who's been there too with charities, I repeat the warning about keeping things simple. Don't make the mistake of the aid workers who installed fancy electric pumps in the developing world, only to see them end up useless for lack of electricity, spare parts, etc. You have to install technology commensurate with the organization you're putting it in.
Does your charity have professional IT services available to fix things? Do they have a reliable network, both internal and external? If they don't have these basics, then don't give them a solution that needs them. How will new user accounts be added? How will they be shut down? Can people gain access to their stuff from outside the office? Will YOU be required to do all these mundane chores, and what kind of response time can you realistically provide?
Through hard lessons, I have come to some principles:
1. The less hardware in the office, the better. Ideally, you want to go serverless. If you MUST have a server, try for one that just serves files. You will still have to set up backups for it, of course.
2. Cloud services are great: professional-level IT services for pennies (or even free). Google Apps solved many problems at once (email servers, access from outside the office, large file sharing). Email was a BIG problem --- lost password, email client configuration, email password management, and of course SPAM. And it ALL went away in one fell swoop when we went to GMail.
3. Use consumer products as much as possible. They are VERY cheap: new computers can be had for under $300. More important than the dollar price, end users have a ghost of a chance of being able to do self-service if something goes wrong. We used to use Mac OS X Server with network logins. Now we use just basic consumer desktop machines with local logins. Whether we use Mac or PC on the desktop no longer matters. If they buy something from MicroCenter and put it on their desk, they can get it working. They just have to connect to our fileserver, to Google Drive, and then point their browser to GMail. They can get this stuff working without me, at least temporarily. Or an easy call over the phone.
4. Move to web-based databases. See above about eliminating servers from your office... CiviCRM is one free possibility, and there are many paid companies as well.
5. Desktop support is the achilles heel of charity IT. You pay top dollars for bottom skills. Anything that can be administered remotely, you can pay a lot less for better skills. And it's often so easy to do, you can just do what they need done during a coffee break. One more reason to go serverless in the office.
6. Remember that your time IS valuable. A new computer is $300. Setting up a new computer can cost a lot more than that, if you don't keep things simple. Free hardware isn't as much of an asset as you'd think at first, unless you have a VERY quick way to turn it into USEFUL hardware with proper stuff installed. Old hardware is deadly because installs are slower, and thus more costly for you in the end than just buying something new that works.
7. Do anything to avoid viruses. Each virus infection requires full rebuilds, see the costs in (6). We've had good luck with Macs, at least, which somewhat justifies their high price. Linux is better on this front, of course, if you can make it work.
8. Tech Soup is a good place for licensed software. But even better is software that doesn't need licensing. It's not just that it's free --- but also you avoid the hassle of managing license keys.