|If you don't have a real firewall, like a sonicwall that has subscriptions for AV/Spam/etc then you have minimum protection.|
All that has to happen is one of the user pcs gets infected to infect the rest of the network.
You also need to understand the game has changed. Malware/virus is just the first assault to get hacker tools installed.
Hackers would use your controllers hard drives for storage and to launch attacks on the internet. This has happened with hospital equipment.
I would suggest your review your security requirements as part of your 5 year plan.
I would keep those pcs AV up-to-date though I would take them off the internet asap.