I've decided to go ahead and pull the trigger. Worst case scenario is a 60-gal., single-stage, 5 hp, splash-lubricated Ingersoll Rand compressor with a cast iron pump. I think that's safely worth 400 of my bucks for a lightly used, well-maintained unit that sold new above $900.
Best case scenario is that it's a two-stage, 80-gallon, 7.5 hp, pressure-lubricated unit with an aftercooler, low oil light, and automatic tank drain. Those retail for well north of $2,500.
The reality is likely somewhere in between. I'm interested to find out.