Yes, you will need to drill your fixture holes.
Nothing. It is made with built in support.
It is purchased separately and takes a Standard, off the shelf drain.
You will need to choose a door that fits a 27.75" Opening and has a MAX height of 66",
The unit itself is 73" at the flange, but you will need to drop your drywall over that flange and finish up to the ceiling. The same with the front flange. You will want to put a bead of caulking between the drywall and the unit to finish off the edge and seal it.
Yes, it can be used with a Shower Curtain.
Apologies, but I wasn't able to reference that SKU number. I have not heard of any issues with Oatey drains before and it should it. 4.25" should work...was it the 4.5"?
Any Shower drain that fits a 3.25" cutout and your pipe connection should work. Typically, any off the shelf item should work, so you can find you Options and Price Point.