I'm also new to this KingCreaky, so can only try help a wee bit...

For this size machine which is similar to the size I'm designing, choosing 1610 ballscrews seems spot on, but I think the gurus are likely to suggest 1605 only for the short Z-axis. And spinning the ballscrew rather than the nut is the default norm.

If you're gonna use supported round rail I think the gurus would suggest Chai's SBR20 rail.

Where peeps do use extrusion, a version of 45x90 extrusion is popular on this forum that hails from your neck of the woods... KJN in Leicester... Aluminium Profile with 10mm Slot

The KJN 45x90 extrusion has slots that are well suited to the precision-profile-rail that is terribly pricey, but not particularly suited to supported round rail.

Hope this helps.