[Help] 3D design/Rendering system

Hi, This is my first post here. I've read some of the post and the recommended builds for certain usages and I want to ask for help when building a system which will be used mainly for rendering and 3D design.
I've built many systems before but I've never oriented them for rendering.

The price limit on this system will be of 2500 €.

I havent built a system for a long time so I dont know which components will be more convinient for this type of built. But I have some general doubts:

- Is it worth (or a good idea) to buy a dual socket motherboard for two i7s?
- Which nvidia family of graphics cards suits better GTX or Quadro?
- Is SLI or TriSLI worth it?

With these doubts solve I can start to think of a suitable build, if anyone wants to recommend a certain build or any post with information that could help me, please feel free to post.

Thanks in advance!