Email server solution/Firewall/DHCP server. A Small data center for a company, how to make a good design for this? What are the Platforms that should be used for each solution?

your Domain Conterroler,DNS AND DHCP you can use Windows Server 2012 R2                                        for firewall you can use free UTM This calling Pfsense                                                                                         and Mail Server you can use Exchange 2010 or 2016 Do not use 2013