Whichever way you decide to go, go with the Windows x Pro 64bit and dump all of the un-needed software from the computer. Use the least invasive internet browser with which you are comfortable and do what is needed to to take control of the system updates. Use an SSD for cache files.
Either get rid of or control all of the things that might want to interrupt Hauptwerk while you are playing and not connected to the internet. Only connect to the internet when necessary to maintain, download or upgrade your system or get remote desktop help. Hauptwerk has a list of things to do and a site called Black Viper also has a list that is more extensive. Update the Windows security on your schedule and avoid features updates that are not useful. Do not use the Hauptwerk computer to surf and keep the virus defense system disabled while playing the Hauptwerk organ but invoked prior to turning on/plugging in the Ethernet access.
Train the computer to power on ready to play your favorite sample set and always shut it down from Hauptwerk using the Shut Computer Down function in Hauptwerk. I use a smart strip AC control that senses the computer is on and turns on the peripherals and audio.
If you have a spare MIDI button, assign it to the Shut Computer Down function. Always backup your Hauptwerk settings after major revisions to computer, console or combinations. NEVER use the System Restore on a Haupwerk computer unless Martin changes his stance on System Restore.
As for an OS choice, if you are a computer person then definitely Win 10 Pro 64bit will give the longest time until EOL. At least 10 years from the intro for Win 10. Many people are talking about how Win 10 is going to stay for longer than that and just upgrade. So maybe it will become Windows XOS
Thomas