Firewall and Antivirus for Windows Servers

Dear all, we are seeking an expert in the domain of security that who can build a robust firewall (preferably to be opensource and based on Linux) and antivirus for Windows Server 2012. We are a web hosting company and our scenario is like the below 1- We have 3 Core Function Servers ( Domain Controller , Microsoft Exchange, and Web Server) 2- Managed by a 4th server that has SolidCP control panel that supposedly managing the previously mentioned servers in point "1" What we need from you? 1- We need to deploy the 5th server to act as a firewall to eliminate any possibility that neither the SolidCP server nor the Core Functioning Servers (Domain Controller, Exchange Server, and the Web Server) would be exposed and stand against any hacking attempts (We prefer to have a robust firewall that is based on Linux and better off to be open source) 2- Antvirus to be installed on all the previously mentioned servers (they are all Windows 2012 R2 server) to scan periodically and instantly to any potential virus or any form of threat 3- Those security layers should never affect negatively on the service flow .. we need to balance the best security practice while maintaining the services flow Good luck for everyone!