First off, I would not plan to have classes running in both rings. If it ended up raining the day of the show and you only had the indoor available you would be between a rock and a hard place, and possibly end up having to refund money. I would have the show in the indoor and warmup can be in the outdoor.
To appeal to both the english and western crowds you could offer english classes in the morning and western classes in the afternoon, after a lunch break or something. On the schedule you could put "afternoon classes not to start before 1:00pm" that way people know when they should get there.
I'm assuming it's going to be a hunter-type schooling show for the english people. Western... idk, like a showmanship? Horsemanship? Pleasure? Thing (haha, I don't ride western). Call up other local showing assoiations to see who they use for judging, maybe you could work out a deal with the judge - see if they'll judge for a reduced cost or totally volunteer their services since it's a benefit show.
Volunteers can be found anywhere :) Try to recruit people from your barn, make it worth their while - "volunteer for 4 hours and get 1/2 off your next lesson". You can also use horse show moms and dads for the easier jobs ;)
Class list - work with the judge to see what they prefer to judge. Most smaller schooling show have jump courses from ground poles up to 2'6", maybe 3'. You can raise the jump height 6" each division.
Advertise - put flyers in the local feed/tack stores. If there is a regional horse magazine/newsletter run a small ad or atleast have it put on the calendar of events, which is often free to do.
Make entry fees affordable, but only set them after you figure out how much you're going to have to spend for the judge, food, ribbons, etc. The shows around here range from 6-20 dollars a class.
For food all you really need is water, soda and some type of gatorade type drink. Chips, nachos, hotdogs and burgers... you know, horse show food ;) Grab a barn dad to man the grill and you'll be fine!
For ribbons check out Horse Show Awards, Corporate Awards, Recognition Awards - Hodges Badge Company
, great, affordable selection. Check out their "horse show in a box" too, might be something you can use :)