I'm in a similar position at an NGO, except that we have offices in 4 other countries with 20 or more people each.
Here's what worked for us, your results may vary:
New desktops: should it be laptops (with dockingstations), regular desktop machines or thin clients?
Laptops. We are frequently traveling, often to areas with little to no internet access, and being able to bring your data with you is a good thing. Mostly Macbooks, as they are reliable, easy to use, and integrate well with the rest of our systems.
Servers: We need a server for authentication and user management.
We use Zimbra for authentication and user management. It also serves our email - IMAP and SMTP, shared calendars and task lists - synchronized over calDAV, and a web-based interface to all of the above.
We also need an internal media server
Each office has an internal Linux server running Samba, authenticating over LDAP to Zimbra. Works equally well with Mac and Windows clients.
Finally we would like to have our web server in house.
Are you sure about that? Do you have the bandwidth and a reliable enough connection? We went with a dedicated server hosted somewhere with multiple redundant connections.
feel free to comment on anything important not on the list.
Email and collaboration software?
Again we use Zimbra, and it integrates remarkably well with iCal on Mail on the Macs. Windows users can use Thunderbird + Lightning or the Zimbra desktop client.
Printing?
We run CUPS on the Linux server, so the Macs pick up the shared printers automatically. Windows users can print over Samba with click to install drivers.