1. If you are not too worried about price, by all means, buy a i9-9900K.
How high you can overclock any processor will be determined by your luck in getting a good chip.
These statistics come from silicon lottery, an outfit that bins chips and sells the better ones for a price premium.
As of 2/6/2019
What percent can get an overclock at a somewhat sane Vcore in the 1.337 to 1.375 range.
And AVX offset = 2.
I7-9700K
5.2 10%
5.1 35%
5.0 78%
4.9 100%
As of 2/6/2019 from silicon lottery:
What percent can get an overclock at a somewhat sane vcore in the 1.275v to 1.312
And AVX offset = 2.
I9-9900K
4.8 100%
4.9 86%
5.0 39%
5.1 8%
One of the 5.1 9900K processors will cost you a cool $900.
Overclocking with higher voltages will damage your processor.
The high numbers you hear bandied about are from those with good chips that have stressed them above safe levels.
3. On cooling, consider the source. NZXT makes fancy aio coolers and want to sell them.
The second source is more reasonable. Note that their recommendation of an aio is for extreme overclocking which I hope you are not considering.
It is also for those with cramped cases.
Your H700 case is a nice one with plenty of front air intake capability.
As to cooling capability, here is a comparison of the NH-D15 vs. several aio coolers.
http://www.relaxedtech.com/reviews/noctua/nh-d15-versus-closed-loop-liquid-coolers/1
The NH-D15s is essentially the same cooler as the NH-D15 except that the D15s is a high compatibility version that clears tall ram and is offset to clear graphics cards.
One can add a second fan for a slight bump in cooling.
The temperatures on an open test bed are very close.
Where an aio loses is in the noise required for maximum cooling.
4. By heavy multitasking, I mean running other batch apps while gaming.
Editing a video for example.
5. Computer parts get better and cheaper over time.
Why pay for something now that you do not plan to use?
If you do buy a HDD, buy an EXTERNAL one for backup.
6. Games will be limited by either the cpu or the graphics card.
If your games are fast action shooters, spend more on a fast graphics card.
If you are planning to use a high resolution monitor, buy a strong graphics card.
If your games are sims, mmo or strategy types, they depend on a fast single master thread.
If you play multiplayer games with many participants, there is where the 16 threads of a 9900K will shine.