It depends on what you're using the pc for. For primarily gaming then no the i7 wouldn't make a huge difference since most games don't show much if any benefit from hyperthreading. Hyperthreading can help keep the cores busy more often in workloads that take advantage of it but it's still a quad core cpu like the i5. The other primary difference are clock speeds out of the box. The 4690k's turbo boost out of the box being 3.9ghz and the 4790k's being 4.4ghz. The i7 typically costs 50% more than the i5 and you definitely won't see 50% performance improvement.
At 3.9ghz I wouldn't really consider that overclocked. 4.2ghz should be easy enough using a budget air cooler like a 212 evo or cryorig h7. Ambient temps will also play a role in cooling so without knowing your room temps it's hard to say about that cooler. Here's a link to some tests done by dvtests regarding the t4 vs the 212 evo, the t4 failed while the 212 evo did pretty well. I'd recommend a better cooler though water cooling isn't required either. An h100i is overkill and overpriced for the cooling needed.
http://www.dvtests.com/cooler-master-hyper-t4-test-and-review/
When it comes to applying more core voltage each chip is a little different. A simple way to go about it is to increase the multiplier a little at a time and stress test for stability using things like asus realbench, intel burn test, p95 version 26.6, etc. Use a program like hwinfo64 to keep an eye on vcore and thermals or you can also use something like realtemp to keep track of temps. If the multiplier is raised and the pc crashes (unstable, bsod) then it needs a little more vcore or core voltage.
As some others have pointed out, 1.3v is where most people are comfortable running haswell or devil's canyon cpu's for daily use. 1.35v is getting near the upper limit of 'safe', personally I keep mine below 1.3v. I have two pc's both running the 4690k, one at 4.6ghz the other at 4.2ghz and the one at 4.2 I didn't even raise the vcore. It's cooled with a 212 evo and even when ambient temps reach 30c in summer it doesn't struggle to keep it cool.
Again each cpu is different but the i5 I have at 4.6ghz isn't even running vcore over 1.3v. 1.35v for 4.2 is either a terrible overclocking chip (poor luck with the silicon lottery) or someone hasn't spent much time fine tuning it. The more vcore the more heat it produces. If you're unfamiliar with overclocking check out some haswell overclocking guides that can give some basic instructions. Something like this for instance.
http://www.overclock.net/t/1411077/haswell-overclocking-guide-with-statistics
The 4690k isn't really all that 'hot' despite core temps. With a decent air cooler and reasonable case airflow and ambient room temps it should be able to handle gaming loads around 60-70c even when overclocked. That's 30-40c below the point where it begins to throttle. If your ambient temps are higher, around 30c or so you may need a better cooler. Any cooler whether air or all in one water cooling rely on ambient room air to cool the cpu one way or another and the higher the heat in the room the more cooling will be needed. Cooler air cools better than hotter air.
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