The best bang-for-the-buck Cinema 4D Workstation currently (12/2015) would consist of the following hardware components:
CPU: Intel 4790K or 6700K
RAM: 32 Gbyte
Graphics Card: Nvidia GTX 960
Mainboard: Asus H97 (Intel 4790K CPU), Asus H110 (Intel 6700K CPU)
HardDrive: Any kind of SSD (Samsung, Corsair)
Many Cinema 4D features like deformers, generators, cloners, are calculated on the processor using only a single-core. This is where you will want the fastest possible single-core speed. The Intel 4790K and 6700K both have 4 GHz and some more Turbo GHz to make the viewport as fast as currently possible.
RAM is quite cheap nowadays and 32GB helps when you have large scenes and textures.
There is no need to get an overpriced Nvidia Workstation Graphics Card. Since the bottleneck in viewport speed is almost always the single-core cpu speed, a “Gaming” Graphics Card sich as the Nvidia GTX 960 will be more than enough. This Card can also be used for rendering with GPU-Renderers such as Octane or Vray 3.25.
This Workstation will have roughly 830 R15 Cinebench Points, which is still very good for rendering. This PC though was configurated as a Workstation and not a render-node. If you are looking for fastest render-speed The Processor configuration will have to go into a different direction: Max CPU Cores (e.g. Dual Xeon E5 2630v3), but this will then be much slower for working in the viewport.