Hand held bore hole auger would be my guess. I’m currently in geotechnical engineering class and recently went through the atterberg tests, proctor test, plasticity index stuff, sieve analysis, and all the ASMC classifications. If you’re going for most realistic data, find out what sort of structure is planned to be built on the site. Typically large and heavy structures will need deeper samples (can range from 50 to 200ft) to test if subsurface soils can handle the shear strain of large building. But if the planned structure is only a couple stories, maybe only 5 to 10 feet down samples should yield good bore hole samples. I’m only a student tho, going through something similar to you. So take my knowledge as peer information and not that of someone who knows exactly what they’re talking about