One thing I forgot to mention: if you are planning to locate speakers throughout the restaurant space and wire them up to a "central" amp, consider using powered monitors instead. No cable length/timing/impedence issues to worry about, and if you plan it right you can connect the speakers directly to your source. Also, you will have an easier time mixing speaker sizes for different parts of the space (a lounge area, for example, may require less speaker than a main dining room).
Yes, you will need AC drops wherever you plan to locate the speakers, but that is usually no problem. Small monitors from Audioengine and Yamaha, larger units from Definitive Technology, Martin Logan, Meridian and PMC; to name a few.