I agree with assembling the ship in space. However, your dismissal of force or thrust required ignores present day realities. Now if we could build a space elevator, then we might be able to do something like you envisage, but that is not possible yet, and might never be. I am not dismissing the need for energy. But the fact remains that energy is not what is limiting us in our space travel desires. The limiting factor is thrust. Because it takes such a vast amount of rocket fuel to give the thrust to lift a payload into orbit. Once in orbit, small thrust can be enough, because there is time for long, slow acceleration. But to get out of Earth's gravity well is the real problem, and that takes thrust. As to refuelling in the Kuiper Belt, please remember that I am envisaging the space city that decides to travel to another star system will already be somewhere in the solar system working, and that may well be the Kuiper Belt. If not, they are likely to be in some other suitable location, such as near Saturn's Rings, or some object orbiting Jupiter. Refuel and leave. From any such location, the sun's gravity will be small potatoes, and a slow steady acceleration from an ion drive engine will be sufficient. The acceleration is related to mass times velocity of the exhaust. An ion drive engine of sufficiently advanced design should be able to generate an exhaust velocity of 0.999 C. At that velocity, the mass of the ions exiting also increases. Thus the acceleration is greater per unit of the mass of the material to be ionised. The NASA scientists who calculated a theoretical possible travel speed of 0.1 to 0.2 C no doubt took that into account. The big thing I do not know, is how much acceleration such a vessel could generate. When I calculated 55 years to Alpha Centauri, that was based on an acceleration of 0.01 Earth gravities for 10 years, and the same for deceleration.