You're actually not answering a basic question. Will you indulge in OCing?
Nothing bad about it, but it's the main advantage of K version over non-K and may (an absolute may) give you an extra year or two before even OCed i5 starts bottlenecking, that'd be really way down the road though.
Also, I assume you'd want to SLI in future to gain more performance, or atleast want dual GPU capability, for that, Z MoBo would be good and K version would be a good compliment to it. Remember, having a Z MoBo and K CPU doesn't mean you must OC, it means you can OC. You can put non-K on Z MoBo too but it won't make much sense as prices of non-K and K are really close in most regions.
You'd however need a good af cooler for OCing (if you do), but you can purchase it as and when you want. In a nutshell, I'd go with Z MoBo and i5-K, for SLI capability and OCing freedom.