1. There are other cases you could look at. I don't know how often you need to carry around the computer, but there are cases with carrying handles like the
Bitfenix Prodigy (comes in mITX and mATX versions that have identical exterior). Other cube-style cases without the admittedly large protruding handles would be the Silverstone
SG09 and
SG10,
Rosewill Legacy W1, etc. All of those support standard power supplies and don't require overpriced small form factor PSUs.
2. The future is hard to predict. It's only been 18 months since the PS4/XB1 launched, but there doesn't seem to be any concrete evidence that future games will require more cores, just a lot of hope and "we should do that". Developers for the PS4 and XB1 are/were both limited to accessing only 6 CPU cores (source: [
PS4][
XB1]) and even then, it's not a given that developers will make use of all the cores available to them, not all games need it either. If you also look at
Steam's hardware survey numbers, the vast majority of PCs out there used for gaming would apparently have dual or quad core processors. Most PC developers have to make the best use of their time, and deliver a game that works on as many PCs as possible. I think that's why the current trend has been to build games optimized for 2~4 cores. There are a few game engines out there that can scale up to 8 or more processing threads fairly well, but still run well on quad core systems. Also, core count isn't everything either, architecture and performance per core still counts for a lot..
There's a lot of talk about what DirectX 12 will bring to the table in terms of improved support and handling of multiple CPU cores, but so far there's nothing that has come of it.. yet.
In any case, it seems unlikely that buying a i5 4690K will bite you in the ass a few years later, games that can make use of more cores won't run poorly just because you "only" have four cores. In general, games benefit more easily from higher clock speed than extra cores because the former directly affects processing speed whereas the latter is something that the developers and programmers have to properly build into the game engine to make use of extra cores.
3. No problem, the GTX 960 and 970 are right up your alley then.
4. Broadwell might be an option for you if you really want the lower power consumption, but considering how there's not a lot of useful difference between the current 4xxx and new 5xxx models, it's definitely cheaper to just get the 4690K or 4790K and just underclock and undervolt it to get similar lower power consumption. With the right BIOS settings, you can keep the CPU in a low power state that will clock up to a higher speed when work needs to be done, similar to the stock turbo boost feature. That can also apply to overclocking.
Looking at the benchmarking results chart, it seems that all of the i7 4xxx models are overclocked, whereas the 4690K result is at stock speed. It seems that the i5 when overclocked should closely match the overclocked i7 results (the i5 4670K definitely does well). I just wanted to say that if you were to buy the i7 processor, you would still have to overclock it to achieve "A level" performance according to the chart.
Anyway, thanks for the consideration, repost your updated build here later on and some of the other thread regulars can take a look at it. I know the others definitely know more about SFF case suggestions than I do.