Hello Lowhead,
For the generator I agree with Nando. 3 phase Induction motors in the 30hp range are cheap relative to any generator you can build or find. Ebay watching for a few months, looks like $300 for a new 30hp motor inverter duty or continuous duty, you have to pick it up, shipping is a killer. Induction generator is safe, if grid goes down, then the field fails and the generator stops producing. You will want to put a detector on it for this, the generator will double speed with no load. Most motors are not made to run double speed...
Generator will need speed-up gears, consider flat belts, new flat belts claim efficiency on par with gears. Belts are easy to change ratio, lower cost than gears. New belts last long, not as long as gears.
Not talking V-belts.
Question about the turbine, your web says Banki is not good due to low head.
Interesting, low head is what Banki is all about, 3 feet and up, where did you read differently?
Like you say an advantage is you can build it.
What about a plastic pipe Banki? Might not last forever, is cheap can test how things work, much easier to change. Later build it out of metal if needed. Banki does use a nozzle, make sure the pipe feeding the nozzle is several times larger in cross area than the nozzle area. Low preforming Banki the pipe to nozzle area is generally the problem. The pressure of the dam should be the same just before the nozzle, any drop will reduce the power available for the generator.
Any thoughts on siphon feeding it?
Yes, good idea, make sure you can remove air that accumulates in the low pressure area of the pipe.
(highest point, will be negative pressure)
DO NOT MESS with the dam structure. Friend just wanted to drill a hole... hours later the dam failed, water leaked around the pipe and washed it out, what a mess.
Have fun,
Scott.