One thing to keep in mind is that some render packages will charge you more for render nodes, so that may add to cost when considering that route.
I also recommend the PC route if you are going to be rendering a lot. The cost of technology will be significantly cheaper and more configurable to your specific needs. We have a few different configurations here at work. The E5 machines are excellent, but for the cost I would go with the 4930. It really is surprising how fast that is for being a consumer grade card. And as mentioned, you dont have to get into server motherboard so you will save some money there. You can get also get some motherboards with 8 RAM slots in them if you would like to upgrade later.
In terms of video cards, most of our machines are on k5000’s, but we do have a few with multiple GTX970 or 980. To be honest, we dont have any issues with any of them. I have yet to run into any errors with anything. However, that may be due to the software I am using, so your results may vary.
We primarily use Max and vray as well as the Adobe stuff. However, we have also ran Rhino, autocad, sketchup, octane, corona, and a few others. I havent loaded up Solidworks on any of the GTX machines, nor have experience in keyshot on them.