Upload & Sell: Off
Not strictly necessary. If there is a Windows program that you absolutely have to retain, you can run Parallels or VMWare Fusion on the Mac and use the Windows OS (or Bootcamp if you don't mind rebooting into one OS or the other). For example, I have not found a Mac program to replace Quicken. So I have used Parallels for years and have Windows 7 running with Quicken. The Mac makes a great Windows computer actually.
Fair enough, but Quicken or the like is largely stand-alone. If you wanted to use something like Downloader Pro to manage photo file imports and renames and so on then it is a lot messier because you'd be trying to use both OSs with your image files, and that just gets messy. Not impossible, but messy. Plus you have to buy an extra Windows licence or stop using it on the old computer.
Also, Windows is far more prone to bad things and it seems a shame to subject your Mac to them if you don't have to. I try to keep Windows away from as much of my data as possible.
I have found that software such as MS Office is not as compatible as I would like it to be or MS would have us believe (though far better than the previous version) and it is not all that friendly when it comes to two-way file conversions.