Have you seen this...

http://www.vwfuncup.com/paddock/presentation.html

...and in my opinion if you're gonna go mid-engine then make use of the space and weight distribution, put in a VW VR6 or Porsche flat 6 plus turbo if you can afford it. Sounds like big dreams and a fun plan, best wishes with your project and I'm sure, as with most of us, money and time are your only hold backs but your finished car would be nice to see on the track.