I'd say they started building it when they had something like twenty to thirty NX-01 -sized starships completed and began to think big. They got the materials up the same way they had gotten up the stuff that became those starships - I'd suspect a combination of very cheap rocketry (helped along by mastery of gravity) and cargo transporters. Some of the stuff no doubt came from the Moon, which was teeming with mines even in ENT; some may have come from the asteroid belt, but the costs of hauling stuff up from Earth might be about the same as hauling it in from the belt.
The station might well be structurally similar to the starships of the time, built from the very same materials. For some reason, Earth engineers seem to love saucers, so it's only natural that they'd place one atop this space station as well. They might have built it in, say, 2160-2190, as an early monument of the Federation - perhaps with the idea that the entire Starfleet or at least its Home Fleet would fit inside the dome at once. The place would also be an orbital city and factory, and the main starport of Earth once it became commonplace to use transporters for moving people around; ships from places that hadn't yet invented an affordable transporter could dock there and have their passengers beamed down en masse. Mainly, though, the place would be a monument.
Timo Saloniemi
