You are getting yourself in a world of pain!
XP users will bitch and moan enough already if they have to use Windows 7 or 8. Giving them Linux would be much worse.
Here are some common misconceptions about end users:
1. They are stupid and only do stupid thing with there PC: Firefox and libreOffice is not the limit to a persons PC usage. They are going to do more complex things even if they don't realize it. They will want to share files over the network, they may want to attach their Camera to their PC, Video Conference, Do some graphics manipulations, even sometimes do basic system admin on their PC, such as updates or putting in a driver. You need to give them more credit then most people do. Linux for the desktop tends to have a doughnut hole in usability. You get Granny Open your program and browse the web. You got advanced user where you can script and program all you want... The hole is in the Moderate user category.
2. Their PC's will work great with Linux: Who really fully checks the Linux compatibility list when getting a PC. Especially if you initially get a windows PC. Even old PC's you may find that a network controller isn't supported, or a video driver never really worked right with that screen. Hardware makers usually make sure their stuff works on windows first then perhaps in Linux if they feel like there is a market for it.
3. Vendors/Customers/Partners will bend backwards to help you keep supported. I am sending you a DOCX with a Macro in it for you to view. Are you really going to have them redo their work so you can view that document. A vendor may give you a crappy convert. The customer will defiantly give you lip. A partner may question you.
4. We don't use Legacy Software: There is always that piece of legacy software that you have that makes porting expensive.