Thanks to all those who have contributed to this thread. From what has been written I think I understand the following.
There is more information and understanding of Macs rather than Windows machines because Affinity has been used on Macs for longer.
16Gb of Memory is a sensible amount but in a Windows machine leave a couple of slots spare in case you want to add more later.
GPU is not a major issue just make sure it can run the resolutions you want to use on your monitor/monitors (I like a dual monitor set up).
SSD is a good idea but be aware of the limitation on the number of writes that can be made.
Number of CPU cores I'm not so sure about but having found the unofficial performance data I think 6 cores rather than 4 is an advantage (insufficient data on Windows machines to be sure, but over time I would expect more and more software to be written to utilize multi core processors).
New AMD processors --- I've bought into "not quite there" technology before -- its never worked out well so I'm going to stick with Intel, unless it takes me so long to spec and order the computer that the AMD bugs are sorted.
Again thanks to all who have contributed and if you have any comments on my conclusions I would be interested to read them.