Answer: Your zip code places you in zone 6A or the coldest part of zone 6, so depending on your microclimate it may actually be as cold as zone 5. A hilly rocky sandy area is going to be very dry soil, so you will need something drought tolerant. Junipers should do well there and there are many different types to choose from. There are many more possibilities such as creeping thyme, Delosperma cooperi, Geranium lancastriense, Sedum spurium, Dianthus, and other hardy low growing perennials. You might wish to discuss the planting site and your design goals and plant selection with your local nursery staff prior to adding the loam, sometimes adding top soil can create an interface problem or it may not be necessary depending on the plant you use. They should be able to help you identify plants that would thrive there based on a more detailed understanding of your planting site and overall design goals. I hope this gives you some ideas.
Q&A Library Searching Tips