The Royal Sands would be my first pick in your situations followed by the other 3 royals. All 4 would be great choices and easy to get (or rent) for next summer likely at the mid season level. Check with MS to be sure about the level though I'm sure it won't be the high season. Start checking air now.
Most airlines start booking at 330 days out, you should check with the ones you are interested in. For kids including teens, I'd recommend the Sands first but all are great picks. For adults, it's more a matter of preference.
There are no regular grocery stores on the island, however, all of the Royals have a fairly well stocked store where you can pick up all the basics. There are a number of grocery stores in Cancun City including super Wal Mart and a couple of Mexican chains, all were were good. The bus was cheap when we were there (maybe 5 peso) and they ran every few minutes. We just took the bus into town and pickup up what we wanted, no big deal. Restaurants are modest in Cancun, about what prices are locally. But certainly nothing like Cabo or many larger US cities. Of course it depends on your choices. We found the Cancun City places much cheaper but you'd be much further than we were as we were staying at VCI which is much nearer town. While on vacation, it is our habit to eat out one large meal most days and frequntly lunch, doing a light easy breakfast and a light other meal like sandwiches or the like. Bottom line is we didn't see Cancun as overly cheap but certainly not expensive.
