I second the idea of having pizza delivered. I worked in a Dr's office and can say lunch is sometimes involves wolfing done a sandwich in 5 minutes and then it was back to work.
I would first call the office to arrange this for them. At our office on Tues & Thurs were the days that the doctor set aside to meet with drug reps. The drug reps always brought us lunch so I would hate for you to buy lunch only to find out that I drug rep has already beet you to it. Our drug reps always scheduled the lunch meeting a week in advance so the receptionist should have a good idea what day would be good for you to provide lunch.