What is the best computer I can put together to make rendering happen fast?

Q: I use Mari, Maya, Mudbox, Photoshop, 3d equalizer and syntheyes among other software.

I need a 12 core pc with a brilliant graphics card and lots of ram. But which combination of what, I don’t know.

Please tell me what to buy to make render times fast and with no lag when modeling a skyscraper or 2 integrated with a HD video.

I hate time lag!

Help!!!!

Ps my budget is £2800