Good for you for fostering! I hope it's an enjoyable and successful experience for you!
One thing to keep in mind when getting an animal from animal control is that you can assume they're sick. 98% of the time it's nothing major, usually just kennel cough (which is really nothing more than a cold). Most illnesses are easily transmittable from dog to dog, so make sure your personal dog (if you have one) is UTD on shots.
More important than that, though, is the fact that a sick dog is probably going to act differently than a perfectly healthy dog. I've rescued dogs that show no symptoms of kennel cough, but they're very laid back and quiet; a few days later they start showing symptoms, and then a few days later (once they're healthy again), you see their TRUE personality!
As far as food, of course I'm going to tell you to feed the dogs a high-quality diet - good food will help build up their immune system and not only help them get over any illnesses they have, but will also help their overall health. You do, though, want to be sure to feed a food that's fairly easy to find, so that you can suggest that the adopters also feed the same brand. For example, it's probably not reasonable that the average adopter is going to feed a raw diet right away, and he's probably not going to special-order food online or drive 50 miles to the specialty store that sells a particular brand. So pick a brand that basically any pet store carries, or even a brand that's specific to only one pet store but there are several locations of that store in your area (example, if there are 20 Petsmarts in a 50 mile radius, it's probably a good idea to get a food that you can buy at Petsmart).
Some food brands give discounts to rescue groups, and some give free bags of food to get you to buy that brand. You can check out the brands' websites for info there. That would be a really great thing for an adopter, too, to get the first bag of food free.