You can extract the key from the old HDD, but in all likelihood, it is for the OEM version of Windows that was pre-installed on the laptop and it won't be transferable to the other computer. If you try to use it, you will probably find that the Windows installation stops working after 30 days. If the Toshiba motherboard was still good and could be transplanted into the Macbook and you could swap the disk drives, you'd be good to go.
I'm not a Mac guy, but can't you run most Windows software in a Windows emulation shell on the Mac? How critical is it to have an actual Windows OS? I am also not familiar with whether the Mac hardware is indistinguishable from Windows hardware. If there are any differences, it seems like replacing the Mac OS with Windows could open up problems it might not be necessary to deal with (like finding Windows drivers for Mac hardware).
If the MBP is working, I would mess with as little as possible to allow you to run the software you want. However, if your objective is to use the MBP as an inexpensive learning lab to see what you can do, and you don't care whether you accidentally crap up a working computer, that's a different story.