Over concrete I usually form up for a concrete curb five inches high, and 4 1/2" thick, which should match the thickness of the walls. If there are no walls in line with the curb, you can make it any thickness you want. I bond the concrete to the slab with a little thinset. For a little extra strength, I usually drop a couple pieces of rebar in. Probably overkill, but it only costs a couple extra dollars.
You don't want to use treated lumber, and even kiln dried lumber is susceptible to moisture migration from the slab, which can cause undue expansion.
You can also use deck mud if you want, I think quite of few installers do that.