buying a fish tank

Low Iron has to do with the chemical make up of the glass, and it's clarity. The less iron in the glass, the clearer it will be and the less distortion you get of the light spectrum.

Yes, 2 holes per overflow is common--one is for the plumbing to the water out of the tank, the other is for the plumbing to return the water to the tank.

The biggest differences between glass and acrylic will be color, clarity, distortion, price and weight. Color, clarity and distortion--good glass will be better than acrylic, IMO. Distortion is a tricky on, as it depends on the shape of the tank as well as the quality of the material, but comparing flat square panes, I prefer glass. Price sort of is a wash--good glass versus acrylic is pretty close for standard shapes, but acrylic wins if you're looking for custom shapes. The acrylic wins hands down for weight though--big glass tanks are heavy

Yes, you will be able to tell the difference--if it's worth it depends on your preference. Regular glass looks greenish at the needed thickness for an aquarium. Low iron glass won't have as much color to it, giving you a clearer view of the tank occupants. I'd have to know how much the difference is before making the choice. Otherwise, $450 is a good price for a 240 gallon tank, even adding $120 to get it drilled.