pfSense is the alternative to the Linux Small Business Server you've been looking for..
Take a look out at the Linux Distro landscape and there are no end of small business server distributions, these are the distros which provide you with a cheap alternative to create a small business server which in turn provides eMail, DNS, DHCP, Web Servers and many other services you'd need on a server when running a small business.
Usually the scenario would be that you use Microsoft's server products to provide your staff with a reliable back end server system, however over the last few years some of the Linux alternatives have risen like cream does to the top. Everything you could do with a Microsoft server can now be done for free on a Linux Small business distro. Zentayal is an example of this type of Distro. Its polished, there are support options, its well documented, it runs on Ubuntu as a core OS.