Sounds good, I'm looking to do a custom install in my Fiat X1/9 with custom dash (full thing) and console as the original is ugly (dated).
For the console I would recommend not using polystyrene as it is hard to form/shape. You can buy "moulding foam" which would suit better. It is essentially polystyrene but with much smaller grains so it is easily sanded, cut, ground etc without taking out large chunks - my dad used this to make up some airplane wheel spats and it seemed to work well.
I'm still investigating how to do my own custom work as I would really like to put in a leather dash and high console like a TVR!
I think I'll focus on getting the engine rebuilt (DIY) and bodykit fitted (DIY). Best of luck in your project though.