My heater pad will have thermistors for each zone as well, do I need the PT100 expansion board and daughter board to connect the two thermistors?
No, because you will need a DueX5 board anyway to drive your additional motors, and with that you get another 5 heater/thermistor channels.
I am planning to use 24v power supply for the motors, what other voltages do I need the power supply to support for the electronics (12v or 5v)?
No other power supplies needed if your fans and extruder heaters are also 24V.
I’m in the US, should I order directly from you or from a US supplier of the Duet3d?
I suggest you order from Filastruder.
What are the main advantages of using a larger PanelDue and LCD screen over the smaller version?
The 5" and 7" versions can display a little more information than the 4.3" version. Otherwise it's largely a question of looks - the 7" looks good on a large printer, but would look stupid on a small one.