Well, for 300 bucks that's a darn good price, the AGA 75gallon's we have are currently around 350, and thats the sale price. Usual price for a 125 tank/stand combo is around 7-800 I believe.
As for the paint, razor blades will do the trick. Just be careful not to scratch the glass.
The scratches on the front could possibly be buffed out, certain places can remove/replace a sheet of glass for a small fee.
Unfortunately, I don't know anything about the metronidazole, so I can't help you out there. But I would say to scrub the silicone vigourously just to be safe, boil some water and hopefully that would kill off any remaining traces of the chemical.