I currently use Ubuntu, with a different DE.
There are many different ones, Ubuntu has a large user base so can be easier to start with.
It has Firefox, Thunderbird and LibreOffice. Libre can open and convert most windows formats including the new xlsx and docx formats.
It's not really Linux I am seeing more of, Mac's are getting popular with customers as is Google's cloud offering.
Regarding security software, I have Wintex and Heitel applications running with Wine compatibility layer.
Compatibility layers are not perfect (ideally the application should be ported), but it saves needing a Windows machine at all for odd bits.
Other options/workarounds include having a Virtual Windows machine inside your OS or having one PC and using a LogMeIn type service, SSVNC or the like.
Basically I try to avoid any kit that isn't cross-platform, anything else I can normally get to run with Wine.
When I switched to Linux a while ago now I started with the intention of dual-booting a windows hard-drive. Within months I was removing Windows from anything I could.
That wouldn't surprise me, a large amount of stuff in our industry is "Windows only" or needs crappy ActiveX controls.
According to there weby the Server works on Linux but the Client is Windows only.
I would expect a client to work with Wine.
Hopefully someone will design a decent cross-platform solution in the future.