re: 600 km
An orbit of 600km with an inclination of 27.5 degrees would be prime real estate for a spaceport/shipyard.
The maximum payload capacity of the Shuttle would have been reduced by the orbit's height; 600 km is about enough for a mass like the HST's; (11 tonnes?) but a pressurized man-tended module would have to be heavier.
That's one of the problems with the Shuttle design
