As everyone else is saying, it would be best to purchase the actual mill... the hopper and accessories can be made easy enough, but the mill would be difficult without the being a machinist/tool and die maker. The rollers would be better to be solid steel, carbon steel would be sufficient, they would need to have an aggressive knurl placed on their surface as the grain husk is hard and not easily grabbed by just a rough surface. If you used steel or pipe tube of schedule 80 or stronger.. it would be sufficiently strong to do the job but it would also need to be knurled.
good luck whichever direction you decide to pursue.