Robert's Grove is a great choice, but I personally wouldn't go the all-inclusive route, at least not for the entire stay. While Robert's Groves' restaurants are very good, I'd tire of eating at the same places. (Don't miss the Saturday night barbecue, though!)
There many excellent restaurants in Placencia village and Maya Beach, in all price ranges, and it's fun to sample them (though if you don't have a rental car the taxi fares add up). Not much close by Robert's Grove. Laru Beya condos are next door, but they don't have a restaurant.
Grocery stores on the peninsula are pretty small, about the size of a convenience store. I guess Wallen's in the village is still the largest. Prices are higher than in the U.S. and higher than in some other areas of Belize, maybe 50% more than U.S. Belikin by the case now is probably high 40s Bz dollars. Barnacle Bill, who has a place in Maya Beach and posts here, will know the exact price.