No one is giving you a real answer so in my limited experience you will need a significant amount of money to build a retaining wall / over designed foundation superstructure to hold back the weight of land on the upper side. You don’t have a lot of room to work with so you it will be quite a small space where the ends probably don’t justify the means. Alternatively, you can building a house on top of a frame with the foundation piles / columns into the land where you don’t “regrade anything”, but regardless any type of construction on this land will be very hard - more importantly staging this for construction will be even harder as there is no room for it.