Why a X99A mobo? Are you building a server? Why not a Z170? Then throw in 64G of DDR4 PC3200 memory.
I wouldn't trust a Corsair PSU, I'd only buy a Seasonic because all their capacitors come from Japan. There's a few websites that only test power supplies... They'll give you the right "skinny".
As far as graphics cards go, I would spend anywhere between $150 and $200 for a it. yymv, and if you are a gamer you probably wouldn't mind paying $400 for an nVidia card (I don't do ATI because they are power hungry - same with their CPUS.) But only a few die-hard gamers will be willing to pay $1000 for a video card, and those who do probably don't mind paying $1000 for an Intel CPU. Heck, I wouldn;'t be surprised if they have spend $1000 for four 27" 4K monitors. (The price of 4k monitors have come down a lot...)
For a full blown system you'd probably want to spend $300 for the CPU, $300 for memory, $100 - $200 for the mobo, $100 for the power supply. When it comes to SSDs you may want to make sure that it has MLC chips and doesn't have a Sanforce controller on it. Probably for the price of one SSD you can probably buy 2, 3, 4 1TB 64MB cache 7200RPM HDs, like the Western Digital "Blues" or "Blacks".
You'd do your research for mobos at Amazon and Newegg, looking at the reviews, seeing what the rating spread is (you'd want 5 and 4 stars to outnumber the 1 and 2 stars, with the highest rating being 5 stars - for example a mobo that has 50% if 5 stars is a mobo that you would want to stay away from; a mobo where 1 + 2 stars is > than 50% you'd want to stay away from.
Once you have a half dozen candidates you'd print out the Certified memory list and price a stick accordingly. So if one mobo will cost $600 for DDR4 3200, for example, and another mobo will cost $300 or $400 for certified DDR4 3200 RAM, then you can safely go with the mobo that uses the lower priced RAM.
Another consideration is that you want Intel NIC chips. If possible you want Intel SATA controller chips. Why? Because of driver support. When it comes to audio you probably want AC1150, or better.
When it comes to computer cases, it is very personal, some liking "way out there" designs, others only wanting sleek designs. What is most important is the air flow, how the drives are mounted, the number of drive bays, whether it can accept one or two front panle fans, whether the exhaust is 120mm or 140mm, and the size of the exhaust fan on top, 120mm, 140mm, (2) 120s, or a 200mm fan. A 200mm front panel fan probably does no good, whereas two 120s will probably cool much better.
When it comes to fans, I prefer Noctua fans - nice and quite, last a long time (do not bother buying sleeve bearing fans) and come in different airflows. You'd want something around 1200 cfm, or lower, as higher will give more noise. Figure on spending $100 on just the fans. My cpu heatsink is a Noctua, too, and I must of spent around $75 for it. What is important is to not block the RAM slots, that you be able to swap them out if necessary without removing the heatsink. There is absolutely nothing wrong with the stock Intel fan that comes with a CPU, so long as you do not over clock it. One buys an overclock mobo just to be able to load the memory profile and set the RAM speed. You price the highest RAM speed, say DDR4 4200 and compare the price to DDR 3200, 2133, etc. You will reach a point where it you will readily decide what is a reasonable amount to spend. Again, would you rather pay $150 for (4) 16GB sticks of DDR4 2133, $300 for DDR4 3200 or $600 for DDR 4200? (DDR4 4200 will shortly come out. DDR2 3200 will cost $350 - $450 for 64GB).
I usually suggest that whatever amount you spend on the CPU that is the amount you should spend on the RAM. Some say the same thing about the video card, but the highest performing video cards probably need beefier PSUs, and quality PSUs tend to cost more. A good place to start is paying about $100 for a reliable, all Japanese capacitor, 650w PSU. Once you start getting to 750w, 800w, 1000w PSUs the price tends to go up accordingly. The PSU that you select will have to have certain size connectors that will guarantee that they can get to all your devices, the CPU, the video card, the disk drives.
Some heat sinks can be mounted such that the exhaust points up, which if you have a top exhaust fan will likely make for lower case and CPU temps. The computer case should also allow the PSU to be mounted upside down so that it intakes inside case air and exhausts it through the back. Why? Dust. If you put the computer on the floor the underside case filter is likely to get clogged and tend to overheat the PSU.
If the case has a side access door you want to make sure that you can route cables easily - some have less than 1/2" available, some are totally useless. There are websites out there that only test computer cases... Some right side access doors allow the addition of a fan to cool the HDs. And some right side panels "hide" the SSDs. Which means that you need to buy longer drive cables, and much mean that the PSU should have long cables to get to them.
At the time of my purchase the Skylake chips were having problems - some tended to break, but my guess is that it depends on the heatsink - which is why I went with a Noctua heatsink - it has round spacers that you install which will not allow the heatsink feet to warp and crack the CPU, kinda like HP heatsinks on HP PCs - they can only be screwed down so far and no further.
The next problem with Skylake CPUs was the power saving problems - that they did not come out of sleep, that they locked up the mobo and the power plug had to be pulled. I tend to think that it is a problem with cheap PSUs, that one should buy a power supply that is certified to be used with Haswell or Skylake CPUs.
The third problem with the Skylake was with it freezing up "during complex graphic operations," like playing a game. At the time it was suggested to turn off Hyper Threading, basically making the i7 into an i5. That was the deal breaker for me, so I went with an i5, instead. (What i7 can work with OSX? Check Tony's suggestions. If it becomes necessary to turn off HT then you may as well go with the fastest i5 CPU.
And yes, chances are that there is a load full of shops that can put your PC together for about $100. Open up your phone book and call all the stores and request a quote. If they even say, "It depends on the mobo and other parts," then steer clear of them because it is very easy to put together a PC. There are plenty of YouTube videos out there to help. What is absolutely mandatory is that one buy an anti-static strap and never, ever, under no circumstances touch the gold fingers, chips or etches on a memory module. Which is why buying RAM sticks with heat sinks on them is most important.
When researching the mobo, as with any other object, do a Google search for that part and add "problems" to the search criteria. You just have to keep in mind that it may be old info, so make sure that you are buying the latest and greates - for example, you'd want to buy an Apple AirPort Extreme but you'd only want to buy the latest AirPort Express wireless Access Points because the Express were firmware dependent. Don't try and say money by buying an AirPort Express instead of an AirPort Extreme.
So, start with Tony's suggestions, write down the model numbers, find what certified memory models they can use, price the memory, then compare. When it comes to mobos, especially new builds, there's little reason to have PCI slots since that is old tech. You'd definitely want at least two PCI-e slots, with one being 16x, the next 8x and the last 4x. Once you start adding m.2 Sata SSDs, ffor example, they share lanes with the PCI-e slots. And if you decide to go with a uATX mobo make absolutely sure that it has 4 slots instead of 2 because 2 slots will mean that you are more likely to pay much more for memory. For instance, if your mobo can have a max of 32GB (Z97 Haswell, for example) then you can install four 8GB modules for $150 but two 16GB sticks will cost you $300. Same goes for 64GB, you can install four 16GB sticks for $300 or two 32GB for $600. (Yes, I know that they don't make 32GB sticks, yet. But when they do they will be expensive.)
