Yeah, a Mac would be well suited for someone with your requirements. That's what I use my Mac for, and it does the job perfectly.
Adium is pretty good for chatting. I haven't come across any problems with it. It doesn't have voice or video capabilities though, so if you do that a lot, I'd recommend aMSN.
Apple has an online guide for people switching from Windows to Mac, appropriately named Switch 101. Have a link. I found it pretty useful when I switched, but other people I've talked to didn't like it. And if worst comes to worst, you could always set up a Windows partition on the Mac. =\
You can actually change the Mighty Mouse so that it can right click. System Preferences ==> Keyboard & Mouse ==> Mouse. You can choose exactly what you want each of the four buttons to do there.
Bookmarks