Alcam

Not heating but rather overheating causes damage to the CPU. 90 degrees although hot, is not a damaging temperature; any temp that your CPU chooses to run under isn't damaging, it turns itself off when a threat is imminent. 
 
As for cooling; you can get air coolers such as the DRP4 and Noctua NH-U14S / NH-D15 that in most cases out perform most consumer grade AIO water coolers that start at an even higher price point. So what I'm saying is with today's air cooling capabilities you can get brilliant temperatures whilst saving money. There's also the bonus of their being little to no maintenance involved with looking after an air cooler over water cooling. This isn't a huge nag just something to bear in mind. 
 
Like you just kinda said, we have no idea what this machine is going to be used for so suggesting a straight up 50%  RAM and SSD storage decrease isn't a brilliant comment my dude. 

There's no need for me to get anything better than a 1060, I don't game above 1080p. The performance at 1080p with a GTX 1070 doesnt really justify the cost. If at whatever point in the future I get a 1440p monitor I will upgrade GPU, but I'd imagine I'll be waiting a couple of years for that. No rush. 
 
As for my old system I would be looking around £400-500. It will more than likely be passed down to a younger cousin or family friend at around £400, that way I know it goes to a good home

Ohhhhhh, my bad.

It would depend on mobo can you control or not. Such info can be found in mobo manual. Same place you find info if headers are true PWM headers or just DC headers with 4 pins. Most of the time they are just labelled to be AIO_PUMP and such. Sometimes some headers receive extra amps to power more than 3 fans.

I forgot that the Meshify C came with 3x120mm mounts at the front as opposed to 2x. It'll definitely be better to have the 3rd intake at the front as opposed to the top.

I finally did it and installed my ram

Are you playing on your lap or in a place where the ports are choked or could get dusty? If so, open up the laptop, clean it (and get some compressed air) repaste the thermal paste and move your laptop to an open, clean desk in an open area, out of direct sunlight, preferably in a room with A/C.
 
That is an extreme approach, but it works

That might be your opinion and preference. But you can't assume it's everyone's. With aor cooling newbies, they see big numbers and think that's only thing that matters. While you can share your opinion on high rpm fans and benefits of them, giving good advice means telling why some others might not like things that way.

What kind of black magic are you using? Earmuffs?

I think others have done a fair job at responding to the rest of the post. So to respond to this bit if you do feel compelled to de-lid (although you really don't have to, but then no one ever has to hehe) I'd reccomend a liquid metal product for placement between the chip and IHS. 
 
Such as; https://www.amazon.co.uk/Thermal-Grizzly-conductonaut-heat-compound/dp/B01A9KIGSI
               or
               https://www.amazon.co.uk/Coollaboratory-LIQUID-PRO-Liquid-Pro/dp/B001PE5XAC
 
For re-sealing the lid you can pretty much use any type of high heat resistant silicone, and you don't need much of it. 
 
Something like this would suffice just for reference;  https://www.amazon.co.uk/TEMPERATURE-SILICONE-ADHESIVE-SEALANT-RESISTANT/dp/B006C65ZCO
 
I personally used silicone gasket sealant for car gaskets and it worked perfectly fine as it basically is just a heat resistant silicone. 
 
And finally to protect your PCB you can just put something like sticky tape around the chip to avoid getting liquid metal on areas you don't want it to touch. There are tons of videos on YouTube that will show you how to do this whole process safely. 

Yeah like WoodenMarker said, have both of your front fans as intake, and your rear fan as exhaust. As a suggestion from me I'd definitely upgrade that CPU cooler from stock if you're going to be running demanding games at 4K. It's just good practice to keep your CPU at reasonable temps under load. Having it throttle and hit 100 degrees will for sure cause damage to your chip. I'm unsure of your location but you can get a respectable air cooling solution for under £50 easily in the UK. Something to take into account. You'd be surprised the performance increase of a system when its running at a temperature it thrives in. Cheers. 

I'm not sure what the numbers are but there's no common cause of cpu failure. They degrade from use and higher voltage / temps can accelerate that but they still end up being the most reliable component in a computer in most cases. It's not something you should have to worry much about.
Why an AIO in particular?
You can hit 4.7ghz with a cheaper air cooler. Some good options include:
Windale 6: https://uk.pcpartpicker.com/product/ntwqqs/fsp-group-windale-6-600-cfm-cpu-cooler-ac602
Macho Rev. B: https://uk.pcpartpicker.com/product/4TCrxr/thermalright-cpu-cooler-machorevb
Since the Windale is a bit taller than the other coolers mentioned so far, it'd be good to double check the cooler clearance allowed in your case.
Many of these high temp accounts are from Multicore Enhancement on Asus motherboards being enabled by default which is effectively an auto oc which tends to be aggressive on voltage.

If you're wanting sub 60 degrees I'd have to reccomend de-lidding and applying liquid metal for the most reliable results. I doubt even the heftiest of AIO coolers could sustain those kinda temps with an OC 8th gen with the stock intel TIM. But I'm only having a guess using my knowledge here, I have no idea of those actual numbers, I'd suggest doing some research browsing other forums and reviews etc. Just my take on it

Not heating but rather overheating causes damage to the CPU. 90 degrees although hot, is not a damaging temperature; any temp that your CPU chooses to run under isn't damaging, it turns itself off when a threat is imminent. 
 
As for cooling; you can get air coolers such as the DRP4 and Noctua NH-U14S / NH-D15 that in most cases out perform most consumer grade AIO water coolers that start at an even higher price point. So what I'm saying is with today's air cooling capabilities you can get brilliant temperatures whilst saving money. There's also the bonus of their being little to no maintenance involved with looking after an air cooler over water cooling. This isn't a huge nag just something to bear in mind. 
 
Like you just kinda said, we have no idea what this machine is going to be used for so suggesting a straight up 50%  RAM and SSD storage decrease isn't a brilliant comment my dude. 

Thanks for the reassurances 

There are better performing coolers. 
I wouldn't even say the NH-D15 is the best cooler overall. I'd give that title to the NH-U14S for very good performance in an excellent form factor. 
Yes, probably. You can get most of the same oc with a much cheaper cooler. 4.7ghz isn't high to begin with.
<60c won't provide any benefit besides slightly better stability for oc'ing. You're fine up to 80-90c at load. High temps will rarely kill a cpu and the 8700k will shut down for safety at 100c.
The DRP4 performs a bit worse than the NH-D15 and is like a quieter NH-D15S.
If you're in love with the aesthetic, go ahead and get the DRP4.
If you'd rather save a bit of money, you can probably get a cheaper cooler. Where are you shopping/located?

well the 'sweet spot' as you put it all depends on what kind of cooling capabilities you have, and what temps you're getting at idle and during load etc. Just know that typically higher speeds require more power which generates more heat. Undervolting is essentially using a minimal amount of power whilst sustaining a high clock speed; this keeping temperatures lower than they typically would be with just a regular overclock with higher voltage. It may sound daunting but its nothing that a quick google search couldn't put to rest. 
 
Put it this way, a dark rock 4 pro would be acceptable to cool an 8700k at 4.7 on all cores. Assuming you have decent airflow in your case. Undervolting this would just mean your CPU still performs at 4.7 but it just runs a little cooler. 

If you're wanting sub 60 degrees I'd have to reccomend de-lidding and applying liquid metal for the most reliable results. I doubt even the heftiest of AIO coolers could sustain those kinda temps with an OC 8th gen with the stock intel TIM. But I'm only having a guess using my knowledge here, I have no idea of those actual numbers, I'd suggest doing some research browsing other forums and reviews etc. Just my take on it

Your numbers are healthy. Its doubtful you'd ever be taxing the PC in every day use in such a way that you'd hit those max temps anyway. 30-40 degrees idle is also spot on. Nothing to worry about. 

If you are certain that the i3-8350k packs enough of a punch to run what you want it to gaming wise, then yes. The hyper 212 is a brilliant cooler for its price I'm sure it will serve you well. 

did you install it into the same colored slot? You shouldn't have them right next to each other, miss a slot then put it in there. 

You probably could, I personally wouldn't run it on less than 400W.