Agreed - cerith and nassarius snails would be great choices for your sandbed.
One word of caution though not to make the same mistake that lots of us have made - at the pet stores, they may encourage you to get a sand-sifting star. True, they do a great job at cleaning the sand bed, but even a small one will run out of food within a short period of time and starve to death - that's the part they may not be upfront about. We have been able to keep our nassarius snails very happy for a long time. However, they usually stay buried most of the time until I feed so I don't see them much. Then they may come out for a little while. Other than that, the only time I see them is at night with a flashlight. But that doesn't bother me - I know they are busy and hard at work.
