punkfolkrocker wrote: I actually have 2 brand new Medion XP PCs ...[circa 2002]...what's the chances they will actually start up and work...???

Pretty good. If the batteries are flat they'll ask for the date every time. There are versions of Linux that would work. Any modern application would run rather slowly - if there was enough memory to run it at all.

If someone was in the market for a top quality specced PC or Laptop sold for specialist audio recording and editing, which would be the most likely version of Windows factory installed today...???

Almost certainly Win 10 home or pro. Avoid Win 10S if offered.

I doubt if the version of Windows makes much difference - unless you need >128GB of memory - see here.

You'd want as powerful a processor as you could afford, with the ability to process several things (channels, whatever) in parallel. But as far as I'm aware all versions of Windows can use whatever facilities the computer has.

If Win 10 pro is only a few quid more then I'd get it. The extra features - e.g. bitlocker - might be useful.