I'd go with the latest i7 it's about 10$ more and well worth it as it'll be supported for longer period of time.
Depending on how much you plan to overclock your system the Hyper Evo 212 is usually plenty unless you plan to heavy OC your system.
I'd go for 16gb of ram as CAD and 3D software do use quite a bit of RAM.
I think 128gb of Samsung 840 pro is more than enough but to each there own.
I'd go with an 850watt PSU like the one I choose just in case you want to SLI or have it a bit more "future proof".
If you plan on using headphones you don't need a soundcard, the MB soundcard is more than enough.
As for keyboard/mouse that's all preference BUT I love the k90/k95/k60 by corsair. They are rated as the best keyboards you can buy. For a mouse I love the razer deathadder 2013, it's an amazing mouse.
Headphones: AD700 or AD700X and get a desk mic. Those are the best positional headphones sub 300+ dollars(The AD700 doesn't have the best bass though).
I don't really have one, it is really getting the right machine. I suppose it is in the middle range area. I have built incredibly expensive machines for my work i.e. well over $7000 for 3D work where rendering time is the key. In the area I now need I am like a fish out of water. Floundering.
No problem at all. With the set up that I showed you, it'll rune everything you throw at it. If you want you can even run the 770s at SLI and you'll have a VERY beast machine. It'll tear through anything you throw at it. You can also change the case to something that suits your taste more but I'd make sure it's full size.
Okay well if you check out the build that I linked in my first comment it'll show the MB and all the other parts. We can go from there if you have any questions, i'd be more than happy to answer them. Also what resolution will you be using for this computer?
Thanks for your patience, recently went away from ASUS used to be a great fan, however had a bad MOB and had all sorts of dramas, so now looking at Gigabyte, used them over the past year and found them to be good. I just have to find the right board, any suggestions ??