Stainless steel rollers would be great, but they should also be hardened (heat treated and tempered would be ideal).
I don't actually have a crusher, YET... I'm actually looking to get one this month...
I would say, whatever size rollers works best, two rollers should be good, unless you really want three. Offer a handle, or ship it with one, but also make it removable and a size that you can easily place a cordless drill onto (I have one with a 1/2" chuck, but some people only have 3/8" chuck drills).
If you could make it so that you adjust just one knob, but both sides of the roller moves, that would be great. Otherwise, have a graduated adjustment knob on both ends of the adjustable roller so that feeler gauges are not needed. Proper knurling would also be a must.
Also have a base with 5 gallon bucket guide spots integrated. With the opening in the base, going directly into the middle of the bucket. A hopper would also really help (5 pound minimum) for holding the grains.
Personally, I wouldn't care if you leave the discoloration given to the steel from the heat treat and tempering process (just make sure there's no scale on it). From my experience, when making knives, that takes some work to remove.
Basically, I'm looking for an improved version of the Barley Crusher item...
Not sure if you could do it for less than I can get the original for though.