The Z-level stuff is so ingrained into me as a default (I started playing just before it was added) that it's amazing to think that some of the most famous DF stories are from before it was added.
Underground river -> Chasms -> Magma -> Bad Things(tm)
The real trouble I had with the old versions was the inability to build walls. As I recall, once you dug something out it was dug forever.
I don't know the minimum off hand but it is probably around 6 normal stone or cave levels, minus the levels used for open sky space and magma sea which I don't know the minimum for. Trees are multilevel now and I don't know that they will grow on a single layer.
All these fixed elements gave the game a specific progression of increasing rewards and challenges, and a guaranteed story arc for the whole fortress.