Comes with 5 CAL's so if you need need to connect 10 systems you only need 5 additional CAL's.
Next thing is that CAL's allow different systems to connect to the Server they are not Work Station OS Licenses but the Licenses to connect those Workstations to the Server. With M$ Server products you need to pay a License Fee for the Base OS and then for additional Workstations that need to connect to the server. It can get expensive.
Each computer needs it's own OS so ideally you need to look at what is required on the workstations. You also need to look at any agreements that may be in place between the Company and Software Suppliers. M$ for instance offers Discounts to M$ only shops.
You could also place some form of Linux on the Server and have it do the job that is required with Windows Work Stations.
The slowest part here is the WiFi LAN Connection. It's going to be horrendously slow if there is the need to pass a lot of traffic over the network. But because you want/need to use a WiFi LAN the read speeds of the HDD shouldn't be overly important for a file server as the LAN is going to be so slow that the Server will not need to be too highly spec ed out.
Col
[i]edited to add[/i]
This may help explain the M$ Licensing Model
http://www.microsoft.com/windowsserver2008/en/us/licensing.aspx