Wait wait wait, where the hell did this "at least half of all auto mechanics are drug users" BS come from? This is quite frankly an asinine statement to make without anything to back it up, and this has not been my experience in the slightest. I'll agree that generic auto mechanics tend to screw things up more than they help, and have experienced this myself the hard way, but that's no excuse for making baseless ad hominem attacks like this. If you have some reliable research to back this statement up, I'll gladly concede, but I refuse to believe 50+% of auto mechanics use drugs without any evidence whatsoever. Some of my best friends are auto mechanics, so I find this statement quite offensive. (And for the record, none of them do drugs.)
Considering they tried to charge you over a grand and did work you never authorized, yet still charged you $500 for the whole thing, I wouldn't trust a damn thing they say. I'm sure they forgot to mention you needed muffler bearings since those disintegrate after a few years
Honestly, don't let them lay another finger on it. Get your D out of that hellhole, take it back home, ignore everything they said, and do your own diagnosis. The calipers aren't available anymore, and I find it hard to believe that it runs and drives fine yet still needs all four replaced. Pads or rotors, maybe, but the calipers? How the hell does it drive if none of them work, exactly? Or a better question: how does it
stop? They're nickel-and-diming you, and considering their blatantly illegal behavior, I'm sure much of what they recommended is not necessary at all, but you wouldn't know the difference if they did it all at once because they got it running, so who's going to know?
If you want a professional other than yourself do to the work, there's two places near your area I recommend: PJ Grady's or DPI. If you want an official DMC dealer to do the work, DMC Midwest is another option, but they are further away from you than PJ Grady's and DPI are. I have heard great things about all three places, so I am impartial to each.