2. What exactly do you look for in a mobo for mining?
I would look for a quicker ROI, so spending less is your goal (while still having the slots you need).
3. How can you determine how much power your GPU will consume during hashing
Its the same as it would for gaming. Look at the GPU specs to see the power requirements on the rails, add some for your processor (power requirement from their website), plus whatever HDD/SSD you use. Add it all up, with some headroom then pick a PSU that can deliver.