Re: your planned specs. That 8-core AMD processor will probably do a good job for you. If you're able to spend more, an Ivy Bridge-series Intel processor will probably be even better, and they tend to be incredibly long-lived, but your current plan should be plenty sufficient.
Video card-wise, I'd recommend something along the lines of this (AMD R7 260X) or this (nVidia GTX 750 Ti). Both will run you between $150-170, and should last you for several years, as long as you don't expect too much after a few years. As far as AMD vs. nVidia: expect to pay a little more for nVidia cards at the same horsepower level as AMD cards go for. AMD cards are usually faster for the money, but have been historically less stable than an equivalent nVidia card. If your rig is mainly used for audio, I'd recommend you spend a few dollars more and get an nVidia card, as it'll most likely save you some headaches during the setup process.
RAM-wise, 12 gigs is probably plenty, unless you're going hardcore with some gigantic libraries. Nexus 2 is a solid purchase any day of the week, especially since there are so many expansion packs for it. Komplete 9 is also a solid choice, as well, though, especially since it's going to be much more solid in terms of the diversity of sounds you get. If you asked me, I'd recommend Komplete over Nexus, for what it sounds like you want. Keep in mind that Kontakt (only one of the many included plugins) is nearly worth the asking price alone, let alone the included Guitar Rig, Massive, FM8, and others. Also, Native Instruments (the maker of Komplete) is an incredibly ubiquitous brand, so you'll have no trouble at all finding free sound and preset banks, whereas Nexus banks tend to be a bit more pricey.
Hope that helps!