I trailered my horse from Indiana to Montana in a two horse bumper pull trailer. I stopped ever three to four hours to let her out for 15 to 30 minutes. I had a water tank that I filled in Indiana that I had in the bed of the truck that way she had water from Indiana all the way to Montana. Something to think about is a half way stop for a day or two to get your horse out of the trailer. I drove strait there it was a long drive. An other thing is Arizona a brand state. I didn't get brand inspections done and had a little trouble once I got here, Call the arizona brand inspectors office to find out what you need to do.
As for a trailer, Personally I like stock trailers, good air flow, and roomy. If money was not an issue I would get a 1 ton ford have a cummins diesel put in it and get a nice goose neck aluminum stock trailer with at lest one divider. I have a 24 foot that has two dividers. I can get nine to ten horses in it. But that's big. If that not what you need go with a 16 foot trailer. You can get 6 horses in a 16 foot but its "tight" four horses do great in a 16 foot stock trailer.
My two cents.
But check into the brand inspection stuff before you go.