Depends on the application and what you consider a medium enterprise to be. What would work for the average 50 user enterprise is very different from a 50 user enterprise that in addition houses their own web applications, needs redundancy, and content filtering. How many systems and servers are there? How much growth do you expect in the company? What services need to be accessed from outside your network? Do you have satellite offices tying in, VPN or otherwise? What threats are you concerned about besides the obvious (do you want to monitor/control web access, add an extra layer of malware protection, etc.)? How critical is uptime? The more details you provide the better. There's thousands of options, and what you need can range quite a bit.