Just attach both grounds together with the shield wire, if it has one. In the case where primary and secondary are on opposite sides of a transformer, both grounds screw down to the casing on either side. If it's metal encapsulated, that should be attached to ground as well. Continuous ground.
Ground and neutral are not, technically, the same. That's why they are different wires.
1) If you're using a 480/240-240/120 step-down transformer, such as the encapsulated Acme or Hevi-Duty, you must use 240V in. Cheap on Ebay - industrial recyclers. Can be more expensive for shipping.
Topaz/ Xentech/ Phoenix are available in 240/120 and 120/120V models. The former can be used with either voltage in. Surplus distributors, Ebay occasionally. More expensive than general purpose, industrial step-downs.
If you have dedicated circuits and you're hard wiring, 240V has some minor advantages. However, minimum code in many areas for 240V wiring is 12/3 with a 20A rating (12 gauge, 2 insulated hot, 1 insulated neutral, 1 naked ground which is not counted as a conductor. That's even if the neutral isn't utilized). If you're splitting that up before the transformer, that can lead to phase issues. If you already have dedicated lines, chances are they're 12/2 and not suitable for 240V. Locating the transformer near the distribution panel solves that issue as well as mechanical hum, if the panel is in an unoccupied area.
For either type, the maximum draw (aside from momentary start-up spikes) must not exceed 80% of the transformer rating. For audio, I prefer closer to 50%.
2) Usually a bad idea. Been discussed a few times on these forums. Even if they're bonded together, which is a must, there might be issues if they are not in close proximity. An additional ground rod for assurance, within a foot away from the original, should be fine, if you know what else is buried there. Of course, I'm just guessing. I'm not an electrician but I stayed at a Holiday Inn last night.
