Welcome to MAAST! Here is a stab at some answers

(1) When buying used equipment, I understand that aquarium lighting can still be "working" and not really be doing it's job. In metal halides, is it the lamps, the ballast, or both?
Mostly the lamps, they have an operating "Life". They may still be functioning, but their output degrades to the point of being detremental to the tank inhabitants. Depending on the bulb "temperature" and manufacturer, that's typically from 9-18 months. Ballasts are more of a widcard. If they are working you are probably OK, but try to determine the age - if over 5 years they are probably a risk.

(2) How do you test for being "used up"?
Simplest is replace them per the mfg specs. You can borrow a light (PAR) meter, but those aren't eal common among reefers

(3) If any element needs to be replaced, what are the do's and dont's?
Not sure I understand this question? There is a complex list of ballast/bulb compatibilities for metal halides. Somebody posted a link to a chart once, maybe they will respond to this post?

(4) Is it necessary to supplement the Halides with CF or VHO?
Most reefers do, but mostly for appearance. VHA, PC, or T5 actinics mixed with halides gives a much better look and shows the color of the corals better.

- Steve

P.S. Good luck with the merger results!