How high you can OC a I5-8600K is determined by your luck in getting a good chip.
as of 12/01'2017
What % of I5-8600k chips can oc
at a agressive vcore of 1.4 or so and delidded
For a stock, non delidded chip, expect less.
And... I would not spend a lot on water cooling when a good cooler like the Noctua NH-D15s will do the job at half the price.