Since you did the research, this makes my reply much easier. ;)
No difference unless you expect to be plugging a lot of expansion cards (GPUs, raid cards, sound cards, etc) into the PCIe slots.
Right now, the sweet spot is the GTX 970 which should be about $500 AUD. The next step up is obviously the GTX 980 which looks to be about 50% more expensive, but it doesn't come with 50% more performance over the 970.
If you never plan for SLI, then the Extreme3 looks to be just fine. Otherwise, you'll want the Extreme4.
The motherboard will fit just fine.
An all-in-one (AIO) water cooling solution would probably be your best bet, and the case you've chosen even states that it's fully compatible with one:
You might be able to get away with a 120mm cooler, but you'll probably want something like a Corsair H105.Quote:
Enables user to build a complete high-end solution including a 240mm radiator liquid cooling system
I normally just look at how much system-wide power is recommended for the GPU and then tack on 100-200W. Assuming you go with the GTX 970, it calls for 500W system-wide; so, 750-850W should be more than enough.
If you're going with the EVO, then check for a firmware update for the drive when you get the system up and running. You may not need the update, but Samsung's Magician software will check that for you.
You'll want to go with 4x4GB rather than 2x8GB so you can take advantage of HW-E's quad-channel memory. If you want to upgrade and increase the amount of RAM in the future, you will most likely be limited to only being able to install another 4x4GB for a total of 32GB.
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It's a Silverstone FT02B-W. You can see it up close in either of my build videos (01 / 02).