Best OS for Mac Pro 5,1 series

I have purchased a used MacPro from a reputable Apple reseller, as my HW computer, mid-2010, 5,1 series. Quad core, running at 3.22 GHz
Key features:
-500 Gb solid state drive
-SD (DVD burner : reader)
-AirPort (wifi/internet)
-Can take up to 48 Gb RAM
-Has 4 drive bays with caddies
-condition of case etc is 9/10
-will install whatever OS we like

Detailed specs at http://www.everymac.com/systems/apple/m ... specs.html

My questions are :
•Which of the most recent OS iterations are the most HW - friendly? Put another way, are there any which I should avoid?
•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?
•Finally, am I missing anything critical here?

Many thanks for your input.

[mdyde]

Hello Mister_Mirabilis,

•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').

•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).