On a septic system you should rethink whether you'd want a disposal at all.

Enzyme dispensers are a gimmick.

On a septic system, a disposer shouldn't be used like you might if you were hooked up to city sewer - no grinding up masses of potato peels, and no scraping plates into it, particularly not meat and fats. However, a disposer may be used with no detriment to your septic if you use it only for the incidental bits that end up in a sink that are difficult to remove otherwise and would likely be washed down the drain anyway.

With that in mind, any Waste King or InSinkErator will work. No need to get a large one, 3/4 hp will do.