I've supported both PC and Mac and like them both for different reasons. Your decision should be based on what you do in the office most. If it's Adobe-based software for graphics design, then you would probably enjoy the switch a lot. But if you work in a bank and need to run Citrix terminal programs or Windows based emulators, I don't think it will be a good plan. Depends on what you do. Of course, I'm stereotyping because I don't know the situation. There is an astronomer I know who uses Mac/Linux.
My recommendation is to pick the most computer savvy person and convert them to Mac first (do this during slow season, so if you're an accountant, this is not the right time). Give them two months to get used to it. The first month will have many surprises,and work will be disrupted as he or she discusses discoveries with all of you.
The second month will have them seeing benefits. Later, have a meeting and decide. There is no reason why you can't buy all Windows machines, or four more Macs after that. Your investment will be fairly small. The first Mac will be a much smaller business risk.
Best wishes,
DK