Get a managed switch that supports VLANs. Preferably from one of the bigger names, like Cisco, Juniper, Arista, HP, Dell, or Extreme. You can get a Cisco 3500 series on eBay for about $100. Get a router that you can put DD-WRT on (unless you have cash to burn on a real high-end router or L3 switch). You might even be able to get a decent used Cisco router on eBay, but I wouldn't know what to recommend. DD-WRT is a custom firmware for consumer-grade routers that adds support for some enterprise features, and has enough features to get you learning.
Set up a server and share out your files to your workstations. The OS is your choice. If you're looking for real-world Linux experience, use CentOS. CentOS is extremely similar to RHEL and OEL, which are major players in the Enterprise Linux game. Put this server on one VLAN. Put your workstations on another VLAN. Put wireless clients on a third VLAN (DD-WRT can handle this). Configure routing across all three VLANS, plus a default route out to the Internet. After you have this in place, you can play around with ACLs to limit traffic between the VLANs.
This will mimic a small business configuration. DD-WRT isn't really used in a business setting, but it gives you features like true 802.1q VLAN support that you don't normally get unless you spend a pretty significant amount of cash on a dedicated L3 device.