I was rather assuming that. Although assembled in space does not help much unless it is actually MANUFACTURED in space. But constant high acceleration requires constant staging. There are a few possibilities for very high Isp engines that make much more sense for LONG voyages, but they tend to have VERY LOW thrust. 1 G for years at a time requires truly stupendous numbers of stages. Even if we assume more efficient engines than the J-2 used in the Saturn and we ARE assuming lower acceleration, you still need to push large amounts of propellant out the back. And as you consume fuel, the acceleration increases as the weight decreases. High acceleration inevitably means that most of the ship is propellant. Which means that the mass changes greatly over a short period of time. Which means lots of stages, each one a multiple in size of the next one.
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