Yes, Cubase will work fine with quad core and six core processors.
Windows 8 is actually great for audio production - there ar e many write ups on what makes it a great OS for audio production. I would go with it. Windows 7 is going to go away, like it or not. Get ahead of the curve. Get win8, and install the $3 application startisback, and you’ll bypass the start screen entirely. If you aren’t running metro apps, you don’t need to even think about it.
Win 8 allows for very in depth tweaking and is much more well suited to SSD operation than windows 7. Additionally, its memory handling is top notch and I have seen it work great for audio production on machines built for clients (and my own machine).
There are lots of factors when building an audio machine, and plenty of DIYs and write ups on how to do so. It comes down to components, and if they are well suited to audio, the type of RAM you use, type of hard drives, if your motherboard is going to play nicely with what you want to accomplish, the front side bus of your motherboard, the video card you choose, the cooling you choose, case style, and etc.
You can do it, take your time, do your research, and don’t skimp (if you can). Good luck!