But we could build a Ringworld, and then we can spin it for gravity, add shadow squares for a day-night cycle, and not have to rely on artificial gravity generators to prevent us from falling into the sun!

True, but if we expanded our scope to all the space dust in the cubic light-year the sun occupies the center of, we will have enough material to build between 10-29 solar systems, complete with their own stars, so that's probably enough material.

Depends how you do it, if its just a thin reflective foil focusing light on a few collectors then we could probably do it inside a century on just the worlds current production capacity. (baring actually getting all the foil there and engineering collectors and so on, just talking making the foil itself here)