Your question mixes up hardware and software a little.
Enterprise Server is a Windows Server operating system version. This version supports high availability using a technique called clustering. All this really means is that you build two or more physical servers. Install the enterprise operating system on each one and if the primary one pukes out the other one takes over seamlessly. This is typical clustering, nothing unique to Windows.
Microsoft offers business class applications through their cloud based Office 360. To take advantage of enterprise do you need to purchase and maintain two or more servers, and the supporting network. Office 360 only requires a PC that can get to the Internet which requires much less support. You will have to do the math and determine which makes economic sense. The advantage of office 360 is that there are no upfront costs for mail, productivity software and storage. You will get a monthly bill, but you don't need to pay someone to manage servers and software in house. You never have to worry about backing up or restoring data. You only have to worry if you can get to Office 360.