Topsoil would provide better water holding capacity to make it easier to keep your turf from entering drought stress. Your plans sound fine. Check with your local Home Depot garden center manager to see if or when they would be getting turfgrasses in this year and what types they will carry. If you aren't able to get Palmetto contact your County Extension Office and ask them for local sources for that grass variety. When you lay the turf make sure and roll it with a turf roller, available from many rental places, to establish good contact with the underlying soil. Then keep it moist with daily watering for a week or so.
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