All I can really say here is thank you for your post!
There's a lot of valid points throughout this thread, and, just like any Mac vs. PC thread on here there is also a lot of misconception which has been pushed forward through stereotyping and marketing ploys for nearly 20 years now.
Ignoring the entire Mac vs. PC debate and focusing on the original topic of integrating a single Mac computer into an all PC office. Wow. While this is possible, get ready for a lot more work. Not only are you going to have the extra expense of the computer system itself (and yes, the guaranteed higher cost of upgrading it so long as you keep and use Mac computers) but you now have to purchase TWO copies of all your software and all future upgrades: one that is PC compatible and one that is Mac compatible. The cost difference there alone is hard for any business to just shrug off.
Next you have compatibility to consider. As has been pointed out above, some modern software out there can pass file types back and forth easily, but there are a LOT that cannot, so you will have to be very careful about making sure what software is compatible with both of your systems as well as what file types work with both systems. Connecting up and sharing information on the network between these two different platforms can be challenging as well, it's not always just plug in and start grabbing shared files like it's sometimes made out to seem so simple.
As a local computer business, I've had a few different offices ask me if they could integrate a Mac computer into their offices just for a particular person. Once I start writing out all the costs involved, the extra steps necessary to try and make that computer system "play nicely" with the rest, and the changes that will have to be made company-wide to incorporate the upkeep, maintenance, software, and compatibility of that one different computer they just are dumbfounded. My only recommendation can be if you're going to go Mac, go ALL Mac systems, or just go ALL PC systems. Yes, it's possible to use both in the same environment, but you have to be careful and you have to know what you are doing. I'd highly recommend if you plan to do this that you also look for a reliable computer technician, either a single subcontracted technician or a trusted computer business, that can help you out because I guarantee his skills will be necessary for making things work right.