All three Apollo Mobile Launchers were reused. They needed extensive mechanical modifications, of course, to change them from "one-holers" to "three-holers".
I was involved in the final 6 months of the refurbishment on the last of the three. We did all the electrical/HVAC/tubing/cryo lines/pneumatics/cameras, etc. My particular responsibility was installing the TSMs [Tail Service Masts] on either side of the orbiter hole, and connecting all the "stuff" inside. Those TSMs were 30' tall, weighed 67 tons apiece, and were set within +/- .01" in all three dimensions at the opening up top.
That "junk" out behind the HQ building that you saw was probably the LETF (Launch Equipment Test Facility), where they tested the Tail Service Masts for the Shuttles.
Just getting those to work right - safely retracting the T-0 umbilicals - took a crew of hundreds several years to accomplish. There are huge dropweights in there, pyro thrusters, snubbers, etc. Getting the umbilical plugs pulled out of the shuttle, retracted (without damaging them or the shuttle), and the clamshell doors closed in the few seconds before they get burned up by the plume was an engineering achievement in itself. Keep in mind that the stack is in mid-"twang" from the main engines firing while this retraction is going on. Lots of shaking and wobbling...
These people who think going back to the moon will be easy if we can only find "the blueprints" don't have a clue. No offence to any FReepers, but it just isn't that simple.