I don't know much about #1 other than there are different manufacturers and some parts are different between them. I have one ball-lock keg that a standard replacement dip tube won't fit in.
#2: The gas and liquid posts are slightly different diameters, and the quick disconnects are different to fit. The threads are sometimes the same, so they can be switched, which usually just screws people up when they then try to push a disconnect on the wrong post. Even if you're not using disconnects, I'd keep them correct because those kegs can be sold if you change your operation. They'll probably keep their value very well.
#3: There are none anymore. Used ball-lock kegs are high in price and varying quality. Especially ordering online where you can't inspect it first, you may get some fairly junky ones. You can get pin-locks that have been converted which tend to be a bit cheaper. You can also buy brand new ball-lock kegs for well under $100, maybe in the $75 if you search. For that price I wouldn't even consider buying used kegs for ~$50-60 for any commercial operation. Used kegs are just too much of a hastle to refurbish.