CPU cooler. Liquid AIO or Air cooler.

Solution
Since your P300 TG case has CPU cooler clearance up to 160mm, how about MSI Core Frozr L (155mm) for black & red theme?
specs: https://www.msi.com/Graphics-card/CORE-FROZR-L.html
pcpp: https://de.pcpartpicker.com/product/cqgPxr/msi-core-frozr-l-713-cfm-cpu-cooler-core-frozr-l

I also have black & red theme in my Skylake build (full specs with pics in my sig) and i too am thinking to replace my Arctic Freezer i32 with Core Frozr L for better aesthetical match.


I also like to know your reasoning behind saying OP should replace Seasonic M12II-520 EVO PSU. Or you miscalculated PC wattage consumption? Which would be RX 580 at 185W +...


https://www.bhphotovideo.com/c/used/1424800/ It's more than capable of cooling your OC'd 1700. It has the right color scheme. It falls within your budget window. It's used but to what extent? Only the fans can wear out so I don't see an issue.

 
Rarraf, wasn’t being smart or anything, built my pc to specs I got on here and nobody told me that the psu would be a problem so I was just interested to know your reasoning.
 
Since your P300 TG case has CPU cooler clearance up to 160mm, how about MSI Core Frozr L (155mm) for black & red theme?
specs: https://www.msi.com/Graphics-card/CORE-FROZR-L.html
pcpp: https://de.pcpartpicker.com/product/cqgPxr/msi-core-frozr-l-713-cfm-cpu-cooler-core-frozr-l

I also have black & red theme in my Skylake build (full specs with pics in my sig) and i too am thinking to replace my Arctic Freezer i32 with Core Frozr L for better aesthetical match.


I also like to know your reasoning behind saying OP should replace Seasonic M12II-520 EVO PSU. Or you miscalculated PC wattage consumption? Which would be RX 580 at 185W + rest of the system about 200W, making total 385W, leaving 135W to spare. Or are you questioning M12II EVO build quality due to it's 80+ Bronze efficiency?
 
Solution
I’m guessing by the fact that nobody has suggested one that air coolers are better value than liquid AIOs. I suppose I’d be better off with air anyway as I’d only be able to fit a single 140mm rad. Think I’ll go with the msi for the looks. Thanks
 

You can put up to 280mm rad at the front of your case if you so desire,
P300 TG specs: http://www.phanteks.com/Eclipse-P300-TemperedGlass.html

As far as AIOs vs air coolers go, you won't gain any cooling performance if you go with AIO over air cooler since both are cooled by ambient air. (For equal cooling performance between AIOs and air coolers, rad needs to be 240mm or 280mm.)
Here are the positive sides of both CPU cooling methods:

Pros of air coolers:
less cost
less maintenance
less noise
far longer longevity
no leakage risks
doesn't take up case fan slots
additional cooling for the RAM
cools down faster after heavy heat output

Pros of AIOs:
no RAM clearance issues*
no CPU clearance issues
takes longer time to heat up during heavy heat output
* on some cases, top mounted rad can give RAM clearance issues

While how the CPU cooler looks inside the PC depends on a person. Some people prefer to see small AIO pump in the middle of their MoBo with tubing going to the rad while others prefer to see big heatsink with fans in the middle of their MoBo.

Main difference between AIO and air cooler is that with AIO, you'll get more noise at a higher cost while cooling performance remains the same.
Here's also one good article for you to read where king of air coolers (Noctua NH-D15) was put against 5x high-end AIOs, including king of AIOs (NZXT x61 Kraken),
link: http://www.relaxedtech.com/reviews/noctua/nh-d15-versus-closed-loop-liquid-coolers/1

Personally, i'd go with air coolers every day of the week. With same cooling performance, the pros of air coolers outweigh the pros of AIOs considerably. While, for me, the 3 main pros would be:
1. Less noise.
Since i like my PC to be quiet, i can't stand the loud noise AIO makes. Also, when air gets trapped inside the AIO (some AIOs are more prone to this than others), there's additional noise coming from inside the pump.
2. Longevity.
Cheaper AIOs usually last 2-3 years and high-end ones 4-5 years before you need to replace it. While with air coolers, their life expectancy is basically unlimited. Only thing that can go bad on an air cooler is the fan on it. If the fan dies, your CPU still has cooling in form of a big heatsink. Also, new 120mm or 140mm fan doesn't cost much and it's easy to replace one. While with AIOs, the main thing that usually goes bad is the pump itself. And when that happens, your CPU has no cooling whatsoever. Since you can't replace pump on an AIO, you need to buy whole new AIO to replace the old one out.
3. No leakage risks.
Since there's liquid circling inside the AIO, there is always a risk that your AIO can leak. While it's rare, it has happened. It's well known fact that liquids and electronics don't mix.
 
Water coolers send the heat to room temperature air just like air coolers. They aren't any "colder" due to that. Unless they're much bigger and have more fans they don't do much for you. All they can do is get closer to room temperature, maybe.The heatpipes in air coolers are actually solid state "phase change" cooling devices. They're actually more "high tech" than a water loop.