In one of the gunsmithing books, either the NRA or Wolfe Publishing one, there is a description of boring rifle chambers. Certainly not a beginner's project, but an option to reaming.
It is not unusual for a custom cartridge project to involve two reamers - one for the sizing die, also used as a chamber roughing reamer, and the finish reamer. Not an inexpensive approach.
While it would depend on how many cases were going to be sized, I cannot see that hardening would be always necessary. If you stop and think about it, how many times would a brass case have to be run into a neck sizing die for enough steel to be rubbed away to make a dimensional difference in the die? Would there be any reason for a seating die to be hardened?
Newlon Precision sells semi finished die bodies machined to accept commercial neck sizing bushings. This could be another option.
I made a 7/8 x 14 base sizing die from mild steel to remove all traces of the expansion ring from .303 brass fired in Lee Enfield rifles, so that the cases would fit the chamber of my Mk. II** Ross target rifle. Works fine unhardened. Have also made tapered expanding plugs to do serious neck expansions. I think surface finish is more important than hardness.