I was told that much of the slop in my steering was caused by a craked pin or shaft inside my power steering box, and that until that was fixed the car is unsafe to drive (because that crack may shear completely resulting in the loss of steering).

Is this a common ailment of older steering boxes, such that I shouldn't be looking for a junkyard replacement...etc?

I see replacement boxes available via google for $285/ea, before I place the order I wanted to check for all the 'common knowledge' about these boxes that I'm not up to speed on been away from the car for too long.

The only reasons that the internal parts could fail are either an impact had shocked them (as in a FRONT end collision) OR the car was ran without power steering fluid a looooong time and the parts got cooked.
$285 for a replacement isn't terrible if the unit works well. I have had great luck with junkyard units though. I check Diplomat and Gran Fury Cop cars first. I look for clean and dry housings that have NOT leaked. I have swapped several and only had ONE leak from the pitman shaft area. I have paid as little as $25 + core on half price day.