Windows server is going to be the best way to "Deploy" windows.
OEM is only for purchase of NEW computers. dont think you can quallify for OEM unless you replacing the motherboard in an existing PC.
Multi User "WINDOWS" licenses dont exist as far as i know. (atleast not on a small scale e.g sub 50 licenses)
in a server enviroment you MUST purchase CAL's (Client access licenses) for every computer connecting to the server. these computers must ALSO have a windows license of their own.
Changing to a Windows server enviroment is going to be expensive and lengthly procedure and your STILL going to have to reinstall the OS on each of the computers.
If you dont have Windows Server experience i would avoid sticking it into someone's working office. put it in a virtual machine on YOUR computer to play with it and work out active directory, group policies, permissions & etc.
You have a small office, just reload each computer with a fresh copy of Win 7
Windows 7 Home Prem OEM 32 Bit @$ 899 USD115
Windows 7 Home Prem OEM 64 Bit @$ 939 USD120
Windows 7 Pro OEM 64 Bit @$ 1199 USD153
Windows 7 Pro OEM 64 Bit @$ 1239 USD158
Windows 7 Ultimate OEM 32 Bit @$ 1699 USD217
Windows 7 Ultimate OEM 64 Bit @$ 1799 USD230