In terms of the brush, I would not go for a really high-end brush because they will be stolen from you and will need to be replaced. Probably just some filament wheel brush from one of the suppliers. Longer bristles will get in the nooks better, but usually the longest bristles are boars hair, and again - I would not spend the money on those for something that will probably need frequent replacements.
As far as the chemical, you really cant fill a bucket with
soap and expect the break dust to come off. Wheels need a higher concentration solution applied directly to the wheel and then the hp or friction - either machine or brush - to loosen the break dust. You really can't fill buckets with chemicals of this concentration, so you cannot expect great results. You also should not use anything with any bite to it since you are letting the general public handle it - same way self-seve presoaks are on the gentle side.
Maybe just use your self-serve rim cleaner, diluted down in a bucket for this purpose.
I'm sure others who have done this will have suggestions, but what I have outlined above is why I don't bother with letting people do this.
Good luck.
Mike