AzureNissi

The last thing I looked at was the EK Fluid Gaming A240 kit... so something of that kind of order; I'm aware that I may not get change from £300 (yes; I'm in the UK) for a custom loop alone - let alone incl. a case - so if I can get a loop & case for... south of £400 (?) I'd be happy.
 
Not looked at the Corsair kit, yet... it being out there rings a bell (Computex?), so I'll add it to my list of options.

I already have an ssd, plus a 1tb hdd, I felt like it was a good balance.
 
I got these parts over the summer (around May) I've been using them a lot over the summer mostly playing CS:GO surf and GTA 5, I have winded down since school but i'm looking at the untitled goose game which looks kinda fun and im realizing now that it isn't a very intense game. I don't run a lot of current AAA games at the moment. Yeah I can run 1080p 144hz ok. I've been trying to tell myself to hold off but now theres a deadline. But thanks for the advice!

Well, if you just got the parts you should use them and see if you like what you have. 
Get some games and see if your GPU can play those games at 1080p 144hz. Games like PUBG, R6, LOL, and CSGO tend to have high fps (esports titles), but if you play games that are AAA titles, you might not get the performance you want. In that case, then look towards an upgrade! 

The decision is entirely based on who Linus&co think would make the dankest YouTube video(s).

6700k is 4 cores, 8 threads. if it is hitting 100c, you're probably thermal throttling, run something like HWinfo64 and run the game, then you can see a min, max, and average CPU temp, and it will say if it was thermal throttled at any point. Your overclock is either too high, voltage too high, or the cooler isn't mounted properly or running properly. there are a lot of variables at play. Random shutdowns can even be PSU related, you didn't mention the wattage on that. I had a 750W that would overheat with my overclocked 7700k and overclocked 2080Ti and would just shut my PC down to protect itself.

Is your display cable plugged into your motherboard, or your graphics card?