Brandon, First thing I would be thinking here is to reduce the amount of turf you have substantially. Reason being, sandy soil means you will be using an enormous amount of water to keep any turf respectably green. Design the yard so you have only the amount of turf you absolutely need and will use, and the rest of the yard gets designed using regionally appropriate plants that will require very little, if any, supplemental water. With respect to your supply specs, you could go either with a standard rotor or MP, however your choice should based strictly on your soil (steady state infiltration rate) and environmental conditions (wind). With the slope and likely windy conditions, I would probably lean more towards MPRotator that a standard rotor. Depending on wind and area being watered, you could space MP3500 at 30 feet along the edges and corners, and MP3000 full circles at 27 feet if needed. If you have turf from top to bottom of the slope, the correct way to irrigate this would be to have at least three hydrozones ..... top, middle, and bottom of the slope assuming full sun on all areas. That means a minimum of 3 valves .... the number of valves you actually need will depend on your final irrigated area and sprinkler choice. You WILL need bodies with pressure regulating stems and check valves. Finally, if you measured 18 GPM at 50 PSI, design for no more than 15 GPM.