I recently upgraded to a 4k rig I built from scratch, you can do better than an i7-6700k. Go with i7-5820, its got more cores and can be clocked just as fast.
Agree that anything above a current $200 card is over-kill for KSP, but since your doing a high end machine....
That being said I have had HUGE gains on unstable mods with KSP. The extra "umph" does help, where ever it comes from, maybe it just extends the inevitable crash due to a higher threshold. I can run RSS, highest resolutions, with 50 some odd mods for a few hours before it finally crashes on the 32 bit version. I noticed a lot more stable experience upgrading from and AMD 6970 HD going to a Nvidia 970 so there are some things to say about upgrading cards. I also have less "weird" voltage issues, but that's hardly KSP's fault.