Well, for starters, this thread will be moved. It should go in the "Switchers hangout" for further reference.
Alot of software that you may be used to using my not have Mac compatible versions. Alot of them will, but some may not. But there is alot of good software out there to replace those programs for the Mac. As far as your sons game, I did a google search, and didnt find any Mac versions.
You can use nearly any HD you want, but you may have to reformat it. It even says "Compatible with Mac and PC" on the bottom of the
product description of the HD you sited.
As for turbotax, I found
this. So it looks like your all clear in that regards.
The iMac is a good machine. But you may have to get used to using a new operating system. I would recommend visiting an Apple store and checking one out. Good luck
[Edit] Also, the link to the iMac isnt working. Just brings up a blank screen