Hi. How old is your daughter/what grade? If she's got an IEP already, wherever you move, the public school district is supposed honor the services a child is already entitled to, until they can do their own evaluation. In truth, you might find that a larger school district offers a self-contained environment with several other children of similar ability level, whereas if you move to a smaller district, they might be more inclined to cluster a bunch of kids of various levels together, in a less tailored setting. I'm not from AZ, so I don't know what Buckeye schools are like.
My advice is to contact a local support group where you'll be moving to (www.thearc.org is a nationwide group that assists families with Downs and other disabilities, so I'd start with them) and network through other parents to find out about the best school and other resources in the new area. I hope this helps.