You may find great success with any of the many flashes employing Thyristors in their circuitry.
Very simple, and these thyristor equipped flashes work very well.

All you need do, is place the flash at an indicated distance from the subject (taken from the distance scale guide on the flash unit), and then using the guide's shutter and aperture settings based on your ISO/ASA speed.

When enough flash reaches the subject, or reflects-back into the thyristor sensor, the flash (specifically, the thyristor) automatically kills/shuts off the power to the zenon flash tube.
When positioned at proper distance and settings, these units also work well as fill-flash units in daylight.

There are many used brands of these thyristor flash units available on the *Bay. Vivitar 2800, 265, & 273 units could be inexpensive entries (to test the waters), as well as offerings from Sunpack.

Go to Butkus Manuals; read the owners/operators manuals for the above mentioned flashes.
You will learn a lot about the capabilities of these flashes.

Here's one:
http://www.cameramanuals.org/flashes...ivitar_265.pdf