Greetings, Paul and Lesley! A road trip that includes a visit to the Iowa State Fair sounds like great fun!
You're wise to reserve a hotel room as soon as possible, as the hotels fill quickly around the time of the fair. Like most cities, Des Moines has a few areas that I would advise a visitor to avoid from a safety or convenience perspective. However, you should be O.K. if you stick with a major hotel downtown or near the interstates in the suburbs. I would use TripAdvisor to see which hotels are recommended by those who have stayed there recently. In general, the hotels in the western suburbs (West Des Moines, Clive, Johnston), Ankeny and a few hotels near the airport (e.g. the Radisson) are newer.
Please let us know if you have further questions.