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Most stepper drivers I have seen are bipolar not unipolar, unfortunately I have not had any experience with unipolar drivers, so can not offer any advice there.

The heat pads are 230V and all wired in parallel (yes expensive), they are controlled using a solid state relay which is controlled with the 12v heat bed wires from the main board.
if you plan on using PLA, you may not need to use a heated bed.

For a large 3D printer I would not recommend using rods, as they become very heavy compared to aluminium box section, more weight means higher torque/power to accelerate and de-accelerate the axles etc.
The Nema 23's are fine if you use aluminium box section, you may need to go larger if you plan on using rods (not sure on rod diameter, but guessing at least 16 to 20mm for this size build would be required for stiffness)

Bed leveling was difficult but still worked fine with this build, I ended using 9x9 proximity probe mesh to get a close enough shape of the bed which warps around 3 to 5mm in places when heated. I also use a 1mm first layer height.