If nothing has been purchased yet, get the proper equipment which is designed for commercial applications, which means that you should be looking at a 70v system.
I think you got reasonably good information above on some considerations. It's worth adding that commercial 70v amplifiers from Crown, Crestron, Extron, and others are all over eBay at some phenomenal prices. As places go out of business, their old, but very good, used gear goes up for sale for well under $100 on amps which can drive the entire show.
For example, the Extron XPA 2001 amp has 200 watts of power, which is plenty for 9 speakers used for reasonably loud background music. Far louder than would be comfortable while dining I am sure. There are some here for less than 50 bucks shipped to your door...
Get the best deals for Extron Amplifier Xpa 2001 at eBay.com. We have a great online selection at the lowest prices with Fast & Free shipping on many items!
www.ebay.com
What must be said: YOU MAY NOT PLAY PANDORA! It is illegal, and if you are caught, you will be fined.
That's not exactly correct. As your business will be open to the public to enter and leave, you may only use commercially licensed audio playback.
Sonos has a page on it:
Outfit your business with Sonos speakers, licensed ad-free music, and an entire suite of monitoring and management controls, all for a low monthly price. Set the right tone for your brand with best-in-class speakers that you can manage remotely with the Sonos Pro Dashboard. Learn more at Sonos.
www.sonos.com
As does Pandora:
Mood Media Pandora for Business
pandora.moodmedia.com
The bottom line is that if you get caught playing unlicensed music in your space, it could cost you thousands of dollars in an instant. So, pay the monthly service fee for the music as required by law.