I'm not sure a generated electromagnetic field would be good for giving consistent grip all over the hull and as for generating the field, I don't think you would use an iron based hull to generate it but rather electrical coils or I seem to remember vaguely that in order to generate a sufficient strong field to act as a shield plasma maybe came into the equation somewhere. And whilst lifting a heavy aircraft out of a gravity well would be expensive so would accelerating a heavy craft. So, even if built in space, I still think ferrous materials would be avoided.Curious - I envisage use of powerful electromagnetic fields acting as a form of shield - not least to protect against radiation in space - so a significant part of the ship would need to be ferromagnetic. However, I would also imagine that would need a protective layer around it - perhaps a composite foam, which may or may not need to be conductive. I also wouldn't imagine us trying to lift materials directly from the planet's surface, but instead mining in space - hence any shipyards as more likely to be in safe proximity to the asteroid belt, rather than in Earth orbit.