IMHO I think the point is that you need to know which wire it should have. I was/am in the same boat as you. I didn't know much about CarPC's and I still have lots to learn.
One thing I have picked up is it's expensive when you get it wrong, so it's best to get an idea of what you need to do before hand.
Wire Gauges -
http://www.powerstream.com/Wire_Size.htm
Fuses are important too. These depend on how much current you intend to draw and how thick your wire is, and how long it runs for.
But firstly IMHO you probably need to establish what features you want, what sort of setup you want and how much you want to spend. From what I remember Corsa's (which I think used to be the Nova?) are small cars so space is a consideration.
There seem to be a number of ways to attach a screen. Considerations are - do you
want to cut a hole in your dash
have enough room,
want a screen mounted on a stand somewhere
or want a motorised display which fits in your standard 1 din slot.
There a lots of options and down to your own preference, the amount of time you have, and the amount of money you have to spend.