Quoted from Lhyrgoif:

About a year ago I spent a weeks time researching all parts needed to build a MM from scratch using a NBAFB as donor game. The result? I would be forced to get parts from 6-7 different vendors all over the world and the shipping + import taxes alone would be a lot. Some parts like the wire harness and backbox tub are unobtainable and would be a pain to find or make myself. The parts cost including donor game and a new playfield amounted to about the same price as buying a playing MM in decent condition and in my calculations I had not included a single screw, nor LEDs and such but only the larger thungs like mechs/lamp boards/ball guides/ramps and similar items. Add to this at least hundred hours (probably more) of work to put it all together.
I came to the conclusion that the only upside with doing a scratch build (apart from hopefully having fun doing it) is that you get a game in great condition with lots of new parts in it.
Sure, you could go with used parts as much as possible but to me putting in all that work and money and still end up with a game in rather bad condition (like a used worn playfield, scratched cabinet and so on) would suck.
You will absolutely NOT save any money compared to buying a used one and call it a day. I would say this insight is true for any scratch build, including white water.