How to properly organize a local network and all the necessary functionality for the operation of an enterprise?

Question

Mail server

Hello.
What is given:
1. Not much more than 100 computers located in the same building, united in a local network.
2. Serves a network of wines serv.
3. There is an old version of the firewall from Kerio, and their own product for the mailserver.
4. On several computers there are programs using various cryptographic information protection tools, vpn connection to a special information server, 1C and Garant-Aero.
What is missing:
1. Antivirus servers, mainly Kaspersky products.
2. There is no stable operation of the mail server.
What I would like to receive, and how, it seems to me, it is worth organizing the process.
1. pfsense as a gateway to the world.
2. iredmail as a mail server.
3. I don’t know what to use next for 1C and Garant-Aero, and some alternative to the file server.
The essence of the question, after some confusion, is to organize the most stable system, with a minimum budget.
And in the course of the question, quite an entry level, which of the systems to put on separate machines, which can be twisted on virtual ones?