Suspended concrete slab

Afternoon all,

I'm working on a job with a suspended concrete slab for the ground floor level. The site is sloping, starting with slab on ground and the highest point off the ground level will be 1200mm. A couple of questions:
1. Is there a minimum ground clearance for suspended concrete slabs like there is with suspended timber framed floors?
2. Would it be cheaper to a) Have a 172mm suspended concrete slab, or b) have a 100mm concrete slab and fill in the subfloor with clean fill (turning the subfloor brick walls into retaining walls.

Thanks