How should I prepare an all-concrete warehouse space before building out?
A layer of leveling compound will result in a very clean look, but it may not withstand the abuse that it will receive in an industrial environment.  The result could be a lot of chipping of all your hard work, making it look worse than before you started.  If you're just patching, make sure to clean out the divot in the concrete to get the sides clean and vertical, if not a little past vertical, so that the patch can't pop back out.

When power washing indoors, make sure you turn the electricity off first.

For painting the walls, a paint sprayer will get the job done the fastest.  Just get some coveralls and respirator.

For the floor, I would look into an anti-skid sealer that will look good, improve the durability, and keep you safe.