Civil engineer here....
As others have said get a geotech report done - this is ESSENTIAL for a sloping site.
If you know someone in the building/earthworks trade you could get them to comment on your plans and give thumb suck estimates (Note: Think in multiples of $100k additional - eg. we spent $60k 10 years ago on cut to fill + import for 40m of retaining with an average retaining height of only 1m - and minimal additional foundation work - you will require a truckload more. And prices have "doubled" + since we got our work done.)
Also - sloping sites are "cheap" for a reason ie. because they are difficult/expensive to develop - incl access, earthworks, retaining, drainage, foundations, structure - are ALL more complicated - which equal lots of $$$$.