Just for fun I created a basket at Dell with a T105 with RAID Sata disks and 2GB RAM. It also seemed to allow me to add a basic 8 port switch and get the whole lot for less than £900 plus VAT.
If you don't anticipate much grwoth in number of users, the T105 would be a decent choice.
Don't forget a backup device and maybe a UPS though as well if its going to get any kind of serious, dedicated business usage.