You don't say if you are required to get your guests to the restaurant or not or how much time you have before during and after the luncheon.....Getting to/from the restaurants in the Gaslamp by foot could consume a significant amount of time and you would need at least three taxis.
The Marriott, Hyatt, and the Hilton Bayfront all have excellent food and are within walking distance to the convention center. They have excellent views and are capable of hosting a group of that size with no effort. A party of 8 or more may be a challenge for many of the Gaslamp restaurants if you want a quiet, private setting - though there are many excellent restaurants in the Gaslamp. A hotel could also create a special menu of three choices for entres, etc. That would streamline the ordering process and fix your budget.