1) its not really a "science" just make sure you have the proper clearances between the bearings and crankshaft. and make sure you also have the correct gap on the rings.
2)depends on what your doing. a) stock rebuild- measure the crank with a micrometer and make sure its within the stock tolerances. b) performance rebuild- id go with a new one just for a piece of mind and reliability.
3) varies from shop to shop. wouldnt know cause i do my own work.
If you pay attention to detail you should be able to build it your self.