Hi jkeiling, I recall reading that ancient Rome was surrounded and cut off from their regular mills, so they anchored boats in the closest river and used paddlewheels to grind grain into flour.
I noticed recently that the local electric and gas companies are getting their reels of wire and plastic gas tubing on metal reels instead of wood. With the cost of fuel, the old wood reels (5 foot dia, 5 foot wide) are heavier, and thus cost more to transport. Also, rather than pay to return the empties, the metal reels were crushed for scrap weight.
They appeared to be perfect for just this type of thing (if 5'X 5' works for you). Buying new steel and paying to have it cut and welded would cost 20X as much as scrap price. I had an idea about using 4WD front disc wheel bearings with the brake disc. Theres a central hole in the spindle for the front axle where a pipe shaft could pass through, but I haven't finished looking into if that would actually work. if you needed more torque, two could be mounted side by side on the same axle. Steel pipe comes in 10' lengths, and the ends can be cheaply threaded at Home Depot.
Flat paddles create a lot of turbulence, a "Poncelet" is an undershot wheel with curved paddles (looks like a circular saw blade from the side instead of wheel spokes) arranged so that water smoothly slides up the bucket towards the central axle, but stops halfway due to gravity, and then as it slides down and out, it imparts a little more energy to the wheel. The sides of the buckets should be enclosed, and the backs of the buckets could be rough as long as the faces are smooth.
http://www.waterwheelfactory.com/history.htm
How about motorcycle sprockets and chains? Bikers buy new ones all the time from a bike shop when the old chains and sprockets get worn. The old ones get tossed, can be bought for a 6-pack of beer.
Best of luck, have fun!