Car rentals are CHEAP on SXM, and the way to go, versus taxis. Unlike Aruba there is not a reliable bus system either.

Regarding grocery stores, lots of great grocery stores on SXM, much cheaper than Aruba for groceries and for liquor. There is a nice full grocery store about a block from RP (Market Garden), a smaller gourmet grocery (Gourmet Marche), closer than that, plus the larger Grand Marche's, which are around a 15 minute drive, and lots of small chinese groceries.
As far as casinos, the casinos in SXM aren't even as large as Aruba, but they do the trick. The closest is Hollywood, at Simpson Bay Resort.
Mosquitoes are a function of how much rain there has been. For RP in particular, the mosquitoes are often worse on the ground floor, so I would avoid the ground floor there for that reason. (Plus you have a nicer view higher up anyway.)