Your bed will need mains power. No way you'll want to try to find a PSU for that. Going to be very tricky to get any reliable thermistor readings from that bed. Once you get that big, RepRap style heated beds might not be the way to go.
You can buy glass in any size. Mirrored glass will be stronger than regular glass. You may be able to find x-strength glass which is also typically thicker.
Going to be nearly impossible to do CoreXY with IDEX. But if you have that all figured out, would be cool to hear your solution.
Both Smoothie and Duet controllers can handle IDEX and CoreXY. Not sure about both.