Mister_Mirabilis wrote:•Which of the most recent OS iterations are the most HW - friendly? Put another way, are there any which I should avoid?
I would suggest using whichever is the latest operating system version that the computer can run. From a brief look at Apple's requirements for the current version (macOS 10.12 'Sierra'):
https://support.apple.com/kb/SP742?locale=en_US
... my understanding is that it would run on a mid-2010 Mac Pro, which I think your Mac Pro 5,1 is:
http://www.everymac.com/ultimate-mac-lookup/?search_keywords=MacPro5,1
Assuming so, I would recommend using that (macOS 10.12 'Sierra').
Mister_Mirabilis wrote:•As well, I'm intending to run dual touchscreen monitors. I'm aware that mac does not support touchscreen natively, but that there is a third-party patch available. What should I be looking for in a video card?
Hauptwerk isn't itself demanding in terms of graphics processing power, so you should just need to get a graphics card that is able to drive at least the number and resolutions of screens that you plan to use with the computer (and that has the appropriate types of ports to connect them).