no single gpu can push 4kx60hz reliably yet. 1440p is a better option. (i currently run a 3770k with a 1080ti with a 4k monitor - i game at 1440p resolution)
that is quite true and i agree, but how likely are you to upgrade again in the near future, given that the 8700k will last for a good few years yet. case in point are all the people who bought 2600k in 2012 and are just now upgrading to the 8700k - that's 5 years (and 6 processor generations) in between.
better to get a 1700 and then overclock - more value for money - and yes, can also pair that with a 1080ti, no problems
most high end, current generation motherboards have these. the gigabyte motherboard in my spec certainly does
Useful to know that nothing really drives 2160P reliably as yet. Sounds like 1440P is best for me.
Good point about whether I will realistically upgrade the CPU or not. In honesty, probably not. Given that I will probably now go with a more sensibly priced monitor it does free up budget for a better build spec.