mrbill if your not sure of the continuous GPM that your pond can support you might want to wait untill you have more data on the outflow.
Since you mentioned building a system from a car alternator and do most of it yourself you wouldn't have alot of money into it and experimenting with it now would not risk much.
For a low head, low flow system you could mount a plastic Turgo wheel on your alternator, the Rocky hydro website has these and more info on small systems.

Good luck and keep us updated on your progress.