I have an Opel Astra 1.4 G (2000), and i want to replace the engine with another one (seconhand) 2.0 Turbo from opel astra coupe. I want to Know in which point i should pay attention more and especially at the electrical part of the car.
What you describe is a major job requiring many hours work and a great deal of experiance. You will also need a lot more parts than just the engine and gear box.
In the long run you will have to ask yourself why you are doing it. If it is just for the increase in performance you may be better looking at buying a higher spec car to start with.