1. What attire are they planning on wearing for the wedding?
Most musicians should have a standard black "concert attire" for both cool and warm temperatures. If you prefer that they do not wear black, you probably should specify the type of attire, e.g. nice dresses, pant suits, shirt and tie, etc.
2. If they are charging mileage, is it for one car only, or for several?
Generally, it is fair to charge for only one vehicle.
3. What happens if a musician gets ill or hurt, and can not attend the event?
There should be something in the contract that the group will provide qualified subs.
4. Will they charge extra if the playing time goes over just a few minutes?
Some groups book by the hour for weddings. An hour may be enough time for some weddings, however, if your ceremony goes over that time frame, you may get charged for an entire extra hour.
5. Have they actually played together in the group, or are they a "pick-up" group or student group?
Although a business may advertise with a photo their string quartet, these may not be the musicians you get. Many groups are actually "agency" groups, and they hire whoever may be available and often will hire students if there regular professional players are already booked.