KiloAlphaIndigo

That's just caulk to reduce vibration and coil whine. It is meant to be there. Quite common to find that in PSUs.
Corsair SF600 600W Disassembly clearly shows that it is supposed to be there.

Just a general reference for "how much wattage do I need" questions that are filling the forum. Check these and you'll have a general idea of where to start for shopping for a power supply, and questions can be diverted from "how many watts do I need" to a much more specific "what power supply should I get within my budget that will power a system that needs XXX watts"
 
Briefing on buying a quality power supply:
 
Shopping for a power supply is not all about wattage. As much as your mind or people might make it seem like so, it's not. It's about multiple factors, however the most important things are the quality of the PSU and the amps on the 12V rail. Not just watts. This is why the question of "how many watts of a PSU do I need" is becoming drowned out nowadays, because it's no longer a large variable
 
Info on PSU quality:
 
Info on PSU 12V rail amps:
 
On with the numbers!
Again, These numbers are established assuming you are using a quality power supply. The overclocks are also based on heavy overclocking, including overvolting. 
 
 
Another way you could get a rough estimate on your own, is take the TDP of the CPU + GPU(s) and add 100W for other components. This should give you a rough estimate of the maximum power draw your PC will require at stock clock speeds.
 
Use these numbers when shopping for a power supply, but if you're not sure of specifically what model to get, there is absolutely nothing wrong with asking! Create a thread in this section that contains your location (country is fine), preferred store or retailer (if any), and your budget. Our community members will be happy to assist you find a good quality power supply that will be quiet, efficient, and reliable. 
 
Hope this reference guide was of help to you if you're looking to get a new PSU to upgrade or for a build. If there's anything that you feel can be added to this, or if you have any questions then let myself know. This thread is a work in progress and I'll do my best to keep working on it so leave suggestions!   Thanks for looking!
 
Changelog:

Make sure to OC the RAM if your Motherboard supports (Memory Try It, is on my MSI and it auto clocks mine up to 3000, and I get extra 12gb/s bandwith and lower latency by 20ms from 2400)

@KiloAlphaIndigo

2x4GB for sure, Dual Channel is much faster than Single Channel, even if one 8gb is 4000mhz, dual channel 2400mhz will still be faster. 

you can go with 1x8gb and upgrade to 16gb later though

There are multiple reasons why you might want duel gigabit ethernet on a board. Especially ITX where you don't have the luxury of an add in ethernet card presuming that you'll populate the pcie slot with a gpu. You can have one port dedicated to an internet connection and another dedicated to internal file transfers over a server. You can team your connection for 2 gigabit if you really need the extra speed. Linus has demonstrated a lot of these in previous videos, I think the 10gb ethernet on the cheap video covers most of them

I actually thought Intel stopped shipping CPUs with coolers that’s why I added aftermarket one ? thanks for telling me that haha
Saved me there so I can look at improving other parts like RAM or SSD ?

Drop the aftermarket cooler since the 8400 (and 7400 for that matter) can get away with the stock cooler and it's the same price
 
PCPartPicker part list / Price breakdown by merchant
CPU: Intel - Core i5-8400 2.8GHz 6-Core Processor  (£143.99 @ Aria PC) 
Motherboard: Gigabyte - B360N WIFI Mini ITX LGA1151 Motherboard  (£87.99 @ CCL Computers) 
Memory: G.Skill - Ripjaws 4 series 8GB (2 x 4GB) DDR4-2400 Memory  (£59.30 @ Amazon UK) 
Storage: SanDisk - SSD PLUS 480GB 2.5" Solid State Drive  (£89.98 @ Amazon UK) 
Video Card: Gigabyte - GeForce GTX 1080 8GB WINDFORCE OC 8G Video Card  (£479.99 @ Ebuyer) 
Case: Corsair - 250D Mini ITX Tower Case  (£83.89 @ More Computers) 
Power Supply: Corsair - RMx 650W 80+ Gold Certified Fully-Modular ATX Power Supply  (£87.32 @ Amazon UK) 
Total: £1032.46
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-05-11 18:15 BST+0100

2 more cores to hold da 1080 beast

Was just about to suggest this case. It's a cheaper build quality than most, but it's still a very sold case overall, there's a lot of good features, pretty good airflow for its size and cna hold a good amount of hardware in it. It's hard to beat!
The 8400 is 100% worth it. 2 extra cores for $10 will go a long way

Cougar QBX looks like exactly what you need. Other than that get an 8400 instead of a 7400.

this is the case that i'm talking about:

https://www.amazon.com/Cougar-gaming-QBX-Case/dp/B00ZV5YKY6