I thought this time though i would go for a turbo cause i have always wanted one....5spd.....P/steer/.....Aircon where is the downside

So all the basic questions i already know and my next car will be copping the works but i have a few sd33t specific questions that i need some answers for.
1. I got 7 to 8 kms per litre out of my 4spd straight diesel, should i get the same out of a turbo??? Or better
2. They are 12v ar'nt they???
3. These cars are getting on now, does anyone see any major problems with a car thats done around the 300k mark. I need to get around 2 to 3 yrs out of it before i end up living my dream and installing an injected v8.
Cheers for the help guys and i promise no questions regarding bodylifts or suspensions lifts will be asked

marc