I have no plans to overclock really, although it could eventually be a consideration if I want to eek out a more time with the PC. I know a 8700k is just an overclock 8700 but it seems to have higher stock/boost speeds as standard.

May need a new copy of windows, yes.

£1000 - £1400 would be a ballpark for the PC. (Hopefully of the cheaper side of this)