If you do it, a few things to think about:
The stuff you see has bronze bearings rather than ball bearings. Ball bearings don't take shock (from rocks in the grain) as well. They also load up with dust from the grind much quicker.
Solid rollers are the way to go. The same rocks will dent up a hollow roller pretty fast. Each dent lets un-crushed grain through and reduces your efficiency.
Bigger diameter is always better. If you have some 3" round stock, that's going to be the thing to use. Spend some quality time putting groves in it.
Gears between the rollers seem to be a "some do / some don't" kind of thing. They make the adjustable spacing on the rollers harder to implement.
Stainless steel is also a good idea if you have it lying around. Wet grinding grain is indeed something you might want to do.
A half inch (or larger) drive shaft is a good thing for pulleys. Anything over 1/2" will make it a bit tough to drive with common drills.
If you are making up the crusher, make up a proper mounting plate for it at the same time. Plywood is *not* a proper mounting plate. While you are at it, make up a proper mount for the hopper as well ...
If you are making up end plates and a mount, the drive needs to go somewhere. Better to allow to much room than not enough.
For the ultimate design, the dust from the grind is a bit of a hazard (fire, explosion, end of the world ...). An air motor would be a *very* cool way to run one of these.
Now how to justify the new mortgage based on the material list for the new crusher...