Buy them. Yeah I know that you’ve got people who want to donate their old PCs to you. But think… if they don’t want them, why would you? Experience has shown me that donated (used) PCs end up costing too much.
On the flip side, you don’t have to spend a bucket load of money either. For most small offices the prebuilt systems you can get from most small computer shops should be your best buy. Check out a couple in your area and see what kind of deal you can get (you’ll be surprised what you can get for around $500 these days).
Just a word of warning… don’t try to save money by getting the “Home” versions of Windows (XP/Vista/7) as some of the network features are disabled.