I set up 14 small offices, ranging from 4 systems (about a dozen employees, two registers, two back office computers) in a retail storefront to 32 in a small mechanical engineering office. In all those cases, I set up a Windows 2003 standard server (I'd use 2008 R2 now, since it's the most recent and has several advantages) to make sure they could do what they need, have decent security without getting overwhelming, and have future expansion in mind.
The big advantage here, and the advantage at the time I did these, is you can get an unemployed, experienced IT worker to set it up for pretty cheap. I used basic desktop machines, except for the engineering office, with a $600 license for Windows Server Standard, and got everything up and running in a week. The server acted as Router, DHCP, DNS, File and print services, and full nightly backups. (I actually have this same setup at home now, serving one file share and two shared printers.)
The server can be a full corporate level server (Dell R310 machines can be had for less than $2000) or a desktop, if you want to save money that much. I wouldn't advise using desktop type hardware if you can avoid it at all.
For the labor, put an ad in Craigslist asking for resumes, definitely do research on all applicants, and hire someone who has experience, but is temporarily out of work. I did the job for $1000 for <10 users. The one with 32 users took me two weeks, and I charged $2000.