See the instructions in the 357 Reamer sticky on how to do it yourself, there's more info in the FAQs on the same subject.
The pilot must fit the bore, a floating pilot reamer may come with additional pilots the can be swapped out to fit the bore, do a chamber cast including the throat and you'll know what size you need, whether you get a solid or floating pilot reamer, solid pilot reamers will cost less, you can also rent a reamer from 4D, Elk Ridge probably has one too, you'll want a finishing reamer.
Throating just depends on how far out you want to seat bullets, I didn't need to on mine since the 45-120 has plenty of case capacity for the biggest bullets even seated deep, mine worked fine with 555gr bullets seated at to the crimp groove, it just depends on the reamer what you end up with.
See the throating link in the FAQs, I throated 3 of my 45-70s about .250" longer than the existing throats which were all very short, use a test round with a bullet seated to the depth you'd like to load to, drop it in the chamber to check, stop reaming when it fully chambers. For the 45-120 you'll need an extension for the reamer since the throating reamer is probably too short.
Tim