Hey Dave - I'm not trying to rain on your parade but want to make sure you're planning what I think you are.
Here it goes:
Your reasoning is that by taking out the A15 and replacing it with a yet to be re-built A12 turbo that you will end up with more money in your pocket for your Sunny wagon with a rusty body?
Building the turbo engine will be a great experience but it's not going to be a money generator. I think the opposite is true and you will actually be reducing the market for your car.
If you are not worried about re-sale and over capitalising then I say go for it.