Many times you will find the 1 piece unit just attached to studs and glued in place.
Depending on what you buy as a replacement surround will determine how it is measured and the tolerances allowed. A 3 piece has a much higher tolerance than a 5 piece. You will find this more benifitial in older homes that do not have plumb walls or corners.

As for the tub replacement, usually have more damage on old floor when motar and ceramic tile was used. I gut the floors, look at subfloor and determine if tub needs to come out at that point.
Sheetrock around tub is just a given that it is being replaced. First foot or so anyway.

Hey your the guy doing it, have fun...