Home Network :: Finding Security Options Behind The Cisco Linksys E4200 Router?

I have a hobby network at home, the structure of which follows:*Modem * - active SPI firewall -- Then the E4200 distributes data via wifi to/between the following machines --*Server - providing services such as http, FTP, VPN, Mail, MySQL, OpenDirectory, Plex media server, etc.*Two other workstations that access their user details/settings from open directory on the server.Now the purpose of this network other than providing standard access to Internet services from within the LAN is to provide access to the services maintained on my server to users on my LAN when they leave it/go mobile.Now with my limited knowledge of networking I have been using port forwarding to direct service requests through the firewall, however I have found with the E4200 that I have a limited number of custom port fields to use (about 15) which is extremely limiting when I have as many potential services that my server can provide.I could move the Server to a DMZ but then dosen't that put all my other services that have no need to be exposed at unnecessary risk.Do I need to invest in a separate hardware firewall and put it after the modem and before the router?