Scada network questions

I have some questions because I'm not super familiar with Scada networking.

My work has a small scada connection with a primary collective,a secondary and several interface workstations. It has currently been running off of a workgroup and hosts file. The vendor wants us to buy another server so they can add a Domain controller, DNS and NtP server to the infrastructure. This seems excessive for the amount of computers.

My question is if all of that is necessary? They have a time sync issue but we could just sync them all to the primary collective as an ntp server right?

But the domain controller and dns? I don't understand that need.

Can someone enlighten my ignorant ass?

Edit: thank you all for your answers! I appreciate the help!